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LIBRARIES S SSIY¥VuRIT > w n 2 SMITHSONIAN INSTITUTION SMITHSONIAN Pek oi ta LIBRARIES NOILNLILSNI LIBRARIES *\ * NOILNLILSNI LIBRARIES NOILNLILSN LIBRARIES NOILNLILSNI LIBR NOLLNLILSNI SMIT SMITHSONIAN NVINOSHLINS S3ZIYVUSIT_LIBRARIE NF AA S/ tinna SMITHSONIAN NVINOSHLIWS SMITHSONIAN NVINOSHLINS S3IYVYEIT LIBRARIES NVINOSHLIWS $ INSTITUTION NOILONLILSNI SMITHSONIAN Wh: S NVINOSHLIWS NOILALILSNI SMITHSONIAN SJIYVYGIT LIBRARIES NOILALILSNI INSTITU Saruye INSTITUTION SMITHSONIAN NVINOSHLIWS INSTITU’ S3a1uVe INSTITUTION SMITHSONIAN NVINOSHIIWS S3I1yuW 4 A BOY ™ ee, SSL ~ AA. “Sy eae ea ed —— a | — a ae ? Ai = S WASH. 5 S\S z ’ ef = OG Zy: OS i ay = ne > = ‘Nase > = a * > re a LILSNI NVINOSHLIWS Sl uyY G11 LIBRARIES SMITHSONIAN INSTITUTION NOILNLILSN! NOILNLILSNI NOILNLILSNI ZARIES SMITHSONIAN INSTITUTION NOILALILSNI NVINOSHLIWS Saluyvuary S S3INVHNSIT LIBRARIES = a S S eee.) ee fe te ‘% = = te a Sf i : - = = E nS a Dc. 2 2 ; 2 ALILSNI SMITHSONIAN NVINOSHLINS S3SIYVYSIT LIBRARIES SMIT NVINOSHLINS S3IYVUYUSIT LIBRARIES SMITHSONIAN NVINOSHLIW NS S SMITHSONIAN SMITHSONIAN ARIES SMITHSONIAN INSTITUTION NOILMLILSNI_NVINOSHLINS S31NVYSIT, ai es ~ “ XY - _ os < a * Was < 4 = a 4 = a ) aed ro) = fe) a pees Fa = Zz Satu¥vyugii LIBRARIES z ‘im z ~ = Ss) = S) m2 e = a = om = 5 2 z 2 5 = = e aa e - a - = - Ww) m wo m 2) INSTITUTION NOILNLILSNI NVINOSHLINS S3IYVYRIT NVYINOSHLINS S3IYVYUEITLIBRA z 2 , z 2 = z ww ma iY ie ear z 8 Ng 5 OGhB % 0g c EQ: Gee = . YS , 3 , — aie = ILILSNI_NVINOSHLIWS $3 1YVYS!I7T_ LIBRARIES SMITHSONIAN INSTITUTION - z= uw = cs ty, Zz tu g : g = VG, : oa C4, ire ae” _ os < Vip y Gos A < x Se [ead hg fe es SA fea) 7 poe ‘ = ~ a: =! Wert ae =} or 2 5 IRARIES SMITHSONIAN INSTITUTION NOILNLILSNI NY¥INOSHLINS S31yuyvuNaIT | ‘ ° — 2) “s AY 9 a 5 7 a Ee 7m ey > roe ie ra 0 — v8] — a — a a - a oe i = z on z . 2 D , 0 tyVvya aes 1B RAR I a eee ee nes E = Ee WN FE OS 5 z g BRC 8 2 g ic a iE SQ z = 2 ee “Doe eee: : : ARIES SMITHSONIAN INSTITUTION NOTINIIISN! NuINOCUTINe es uyuar’ | age oF THE NEW YORK ACADEMY OF SCIENCES Val XXVIII pp. 51-166, pll. EXIM | “ Editor, Ranpn W. Towser f. on a's MEMOIR ON THE PHYLOGENY OF THE. 3 8 eo JAW MUSCLES IN RECENT AND ae feo >” FOSSIL, VERTEBRATES - 24 ” 7 y BY CM a ae oe LEVERETT ALLEN ADAMS Sarees rid ; ae giveee O32 Se e ve ~ a by , 4 Are +7 a r rut < bao _ - NEW YORK PUBLISHED BY THE ACADEMY ee = ee 15 January, 1919 os ¥ Se alle Bly Be a ah ee eekly > + «4 Le Sta. Se . age Be * eee fw eae p aa ¥ us Orricers, 1919 ip, ee _ President—Exnzs?. Evisworra Smrru, 50 Rast 41st Street on ‘Vice-Presidents—GBorGE B. ProraM, Franx H. Pixe,.. _ af x ea Epmunp O. Hovey, Pury E. GopparD — sige Bs _ Corresponding Secretary—Hrnry E. Crampton, American M ~ Recording Secretary—Rauru W. Tower, American Museum — Treasurer—JouN TaTiLocK, 37 Wall Street Librarian—Ratru W. Towrr, American Museum : _Editor—Ratru W. Tower, American Museum , ae . ‘ 4 fa. ee es ua), ae SECTION OF ASTRONOMY, PHYSICS AND ) CHB - Chairman—Gnorce B. Pecram, Columbia University = “4 _ Seeretary—K. sauces Faux; Harriman Research Laboratory, . eens. ot Boner ai aie SBOTION OF BIOLOGY = R A iatenan — Paint H. Pixs, College of Physicians and Surgeons Secretary y—WILLIAM K. Grecory, American. Museum ite Ne eater OF GEOLOGY AND | MINERALOGY b Pisin bowond O. Hovey, American Museum Secretary—CItaR.Es | C. Moox, American Museum SECTION OF ANTHROPOLOGY AND psyenoLoay Chairman cP uur E. Gopparp, American Museum ee ei Secretary—Rosert H. Lowrie, American Museum ss Natural History, 77th Street ‘and Chetes). Park, West. - er tae [ANNALS N. Y. ACAD. Scr., Vol. XXVIII, pp. 51-166, Pls. I—XIII. 15 January, 1919] A MEMOIR ON THE PHYLOGENY OF THE JAW MUSCLES IN RECENT AND FOSSIL VERTEBRATES ? By L&EvErRETrt ALLEN ADAMS (Presented before the Academy, 8 March, 1915) CONTENTS . Page Ram U TNC TAMU PERT oR relaahs siele cis sisis iis idl Wis lye's a esse a6 aw dears Obs gues Ae 52 mR PELCE TCC femiTelLmmP TO DLGII 5; «5:0, 00:0. Sino) 0 oid wee 0a, & alazare chavedte eiele tueee 5D Pacer Ea NN TEC) TINO OY on cs..c Greve 315.801 3, 916) 0 616, Fis wrv~ciocs @ aue-> oe elelle gee, ore tee 5D RPCER DISCUS AVE Peay A NAA fas oiet Shaws 18 lve ee ares: Sie rvate Spans dee 5D MeN RNACME CEL UNIO] ViPMet Ake in) sy/aes tore vos) 6 + oii.a ud ay oe teieo ve'w'e ehtkove aS mente ocereR ES 56 SRNSIE RUM SATT CMRTTES OT UT OTM: ci, fatie. os oid a cianeia’ OGDDOGObOCd Hose Sm ADCODAF 115 SOLERO ON. 6 is biafokes wo wheels wre eevee el eleheveies shctele oteloten set kame etek eneaet neaee 116 ee A ee nn Sai aiG Gye don boon noboo oud ND OO0d Tali CUES acu. wre c tM ev ae eatighe evens le teliovee¥eie ta etete sieveh eho eleva heat teen tek temo eee 118 TF 00/10 FRIED ROS OC ES OG AOI OD OOD OOO COS eho dl caus” na) Reconstructions of the jaw muscles in certain extinct vertebrates........ 120 PLACOD EVN S ocovii ote aieleceteie oudia else. blate Joera ela at eu sis.t) ole sel Retaiete ett eens 120 (DS THA DAM cist 6on GOCE OO CC Ob GU SCR SOMAMDO ORDO OCO0G0De60002650900 123 FET YODS coc 8 n bak m5 )a5e otto eens 5.8) oi aosaho ence % Ales oF © aus @'e) aeheV rial a eh ee 128 DIA OSGUTUUSS rows wave. we nie tee oie Susie ssl a0. oe ) Capiti-mandibularis medius (C. m. m.). —= Temporalis, Osawa (superficialis). (c) Capiti-mandibularis profundus (C. m. p.). = Pterygoideus externus, Osawa. Pterygoideus anterior (Pt. a.). MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (D. m.). == Parieto-mandibularis, Osawa. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Capitt-mandibularis.—This muscle is the large muscle mass of the rep- tiles and it is taken to represent the mother mass of several muscles, as is mentioned elsewhere in this paper. It seems to be the conjoined masseter and temporal fibers, so that the divisions of the muscle are given the names used below. (a) Capiti-mandibularis superficialis (C. m. s.).—This part of the temporal mass consists of the fibers that always extend over the deeper fibers in the reptiles. (b) Capiti-mandibularis medius (C. m. m.).—This is the inner part of the main mass, or the temporal of Osawa. The capiti-mandibularis mass arises in the temporal fossa on the parietal, squamosal, quadrate, the inner side of the jugal and from the temporal fascia. It is inserted on the point of the coronoid and on the inside and outside of the posterior third of the mandible. (c) Capiti-mandibularis profundus (C. m. p.).—This deeper section of the temporal mass is separated in this form from the rest of the tem- poral mass. It arises on the parietal, postfrontal, prodtic, epipterygoid, the membrane between epipterygoid and the prodtic and from the outer face of the pterygoid. 92 ANNALS NEW YORK ACADEMY OF SCIENCES Pterygoideus anterior (Pt. a.).—This short, strong muscle arises on the ventral border and inner side of the quadrate, from the inter-orbital floor, pterygoid and transverse. It has the typical reptilian insertion of this muscle, wrapping around the posterior end of the articular. It is inserted on the medial face and ventral border of the posterior fifth of the mandible. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (D. m.).—This muscle, the homologue of the depressor of other animals, is called parieto-mandibularis by Osawa. It is a large muscle which arises from the posterior border of the parietal, from the ligamentum nuche, and to a slight extent from the squamosal. It is inserted on the posterior face of the articular or on its retroarticular process. ALLIGATOR Plate VI, Figs. 5, 6 In the alligator skull there is a great reduction of the posterior region and a corresponding elongation of the antorbital region. The palate is closed by the approximation of the secondary shelves of the maxille, pala- tines and ptervgoids, so that the condition partly resembles that of the mammals. The pterygoids are also very closely appressed, so that the posterior internal nares are forced far to the rear and are almost ventral to the articulation of the mandible with the quadrate. There has been a progressive reduction of the temporal fenestrae, so that the heavily roofed skull of the alligator represents the specialized modern form of an ancient order that had a much more open skull with large fenestre and very different proportions of the skull. The supratemporal fossa has been reduced until it is almost rudimentary, while the rest of the posterior region has also been encroached upon until the muscle space is quite small. The quadrate has shifted its position by inclining posteriorly, civing the animals a much larger gape; the quadrate is fixed or moni- mostylic. The auditory region is much changed by the secondary clos- ing of the otic notch, so that the meatus appears to be in a different posi- tion ; but this condition is merely the result of the closing in of the notch, so that its original position is masked. The small size of the posterior region of the skull and the corresponding reduction of the capiti-man- dibularis is in a way compensated for by the great development of the pterygoideus anterior muscle which has extended over the floor of the ADAMS, PHYLOGENY OF THE JAW MUSCLES 93 palate and into the maxille. This muscle bas an important part in the closing of the long mandibles. The jaw muscles of the alligator are of the type found in monimostylic reptiles. The fixed pterygoids and palatines demand no muscles for their movement and consequently the muscles that function in Varanus and other streptostylic forms are not differentiated. The muscles of the capiti-mandibularis are pinched up, especially in the temporal section, as the temporal fossa is much reduced in most modern Crocodilia. This reduction is secondary, for the early fossil forms show no such reduction, but rather the opposite condition, with large supratemporal and. latero- temporal fenestre and small orbits. Most of the Mesosuchia show a large supratemporal fenestra as in T’eleosawrus, where the fenestre are of large size. There is a progressive closing of this fenestra from Teleo- saurus of the Jura through Geosaurus, Goniopholis and Tomistoma. This closing and the pinching in of the temporal region means that the outer, or masseter, slip of the capiti-mandibularis (capiti-mandibularis superficialis) is enlarging and is taking over the principal work of closing the jaws, while the temporal slip is being progressively reduced. The pterygoid muscle is peculiar in that its anterior portion (pterygoideus anterior) has dug its way under the eye and into the maxilla, extending to a point far anterior to the eye, since it lies on the palatine and pene- trates far into the maxilla. The depressing of the jaw is done by the usual muscle, the depressor mandibule. The Crocodilia spend much of their time lying on the mud, and the action of the depressor in this posi- tion is to raise the head if the mandible is resting on something fairly firm. The Crocodilia have some peculiar external ear muscles, necessary in the crocodile from the advanced condition of the ear with its external flap. The nerves divide the muscles into two groups. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y,) Capiti-mandibularis. (a) Capiti-mandibularis superficialis. = Masseter portion. (b) Capiti-mandibularis medius. = Temporal slip. (c) Capiti-mandibularis profundus. = Deep part of temporal mass. Pterygoideus anterior. == Pterygo-mandibularis, Bradley. Pterygoideus posterior, Lubosch. Q4. ANNALS NEW YORK ACADEMY OF SCIENCES MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Capiti-mandtbularis. (a) Capiti-mandibularis superficialis (C. m. s.)—This is the outer slip of the temporal mass, which does most of the work in the closing of the jaw. It arises on the parietal, squamosal, quadrate and quadrato- jugal and is inserted in the suprameckelian fossa, extending forward into the dentary. It fills the region of the skull behind the pterygoids. The large external fenestra of the mandible gives the muscle room for expan- sion. The superficial layer (masseter) is subdivided into several layers, one being attached to the surangular. (b) Capiti-mandibularis medius (C. m. m.).—This is the smaller of the shps of the mass. It extends down on the inner side of the capiti- mandibularis mass, arising on the bones surrounding the supratemporal fossa, into which it extends. It arises on the parietal, squamosal and postfrontal. Its insertion is on the coronoid, where it curves around the anterior edge of the capiti-mandibularis superficialis and extends under this slip to the mandible. The mandibular nerve separates it from the superficial ship. A small capiti-mandibularis profundus (pterygoideus post. Lubosch) attaches to the tendon of this muscle. Lubosch (1914, p. 699) says: Sehen wir vom Masseter und Pterygoideus anterior ab, so inseriert der sehr schwache, zweischichtige Temporalis mit einer kriftigen kurzen Sehne am Com- plementare. Ein Teil der Fasern endet an einer knorpelharten Zwischensehne. Von dieser Zwischensehne entspringen fraglichen Muskelportionen, welche ihrerseits zum Pterygoideus posterior gehoren. (c) Capiti-mandibularis profundus (C. m. p.).—This small muscle which joins the temporal mass is given this name, as it is assumed to belong to the inner laver of the temporal mass. Pterygoideus antertor—This anterior slip arises on the palatine, maxilla, pterygoid and transverse (ectopterygoid), extending over the floor formed by these bones and digging into the maxille. It extends inward to the limits of the narial passage. This muscle is the same as the great pterygoid muscle in Chelydra and is typical of the monimostylic reptiles. In the crocodile, however, it has a much greater forward ex- tension. ADAMS, PHYLOGENY OF THE JAW MUSCLES 95 Pterygoideus posterior—This muscle (called pterygoideus posterior by Lubosch, who also includes a small slip that joins the capiti-mandibu- laris), arises on the quadratojugal and quadrate and joins with the an- terior slip to be inserted on the mandible. A small slip from this joins the temporal mass and is designated as the profundus in this paper. These pterygoid muscles wrap around the posterior end of the mandible, where they swell out, making a large mass on the postero-ventral side of the jaw. This enlargement of the muscle at its insertion is paralleled in Cryptobranchus, as observed by Lubosch (1914, p. 698), who says: “Der Pterygoideus, zerfallt seiner Innervation nach wiederum in zwei Kom- plexe, der Pterygoideus anterior und der Pterygoideus posterior, worin Ubereinstimmung mit Verhiltnissen der Urodelen besteht.” MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule.—This is the typical depressor of the reptiles, arising on the parietal, squamosal and exoccipital. It is firmly inserted on the posterior prolongation of the articular. IGUANA Plate VII, Figs. 1, 2 The skull of Iguana is of the streptostylic type, although it is quite solidly constructed and much less specialized than that of Varanus. There is very little movement in the quadrate and pterygoid region, so that the muscles that move these parts in Varanus are not differentiated. The rest of the jaw musculature is very similar to that of Varanus, which is more fully described under its section in this paper. The fenestration in the temporal region of the skull of Zguwana is similar to that of Varanus, with a large supratemporal fenestra and with a large sinus below the postorbital and squamosal, this giving the muscles plenty of room to expand. The skull has accommodated itself to the muscles in several ways. The supratemporal fenestra is large and the parietal has expanded dorsally and posteriorly to make a large origin for the capiti-mandibu- laris. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Capiti-mandibularis. (a) Capiti-mandibularis superficialis. = Masseter slip. 96 ANNALS NEW YORK ACADEMY OF SOIENCES (>) Capiti-mandibularis medius. = Temporalis. (c) Capiti-mandibularis profundus. —= External pterygoid of Mivart. —= Pterygoideus externus, Hoffmann. = Pterygo-mandibularis, Hoffmann. —= Internal pterygoid, Sanders. —= Entopterygoid, Sanders. —= Pterygoidien externe, Cuvier, Dumeéril. Pterygoideus anterior. = Pterygoidien interne, Cuvier, Stannius. —= Pterygoideus internus, Hoffmann. == Internal pterygoid, Mivart, Edgeworth. = External pterygoid, Sanders. = Ectopterygoid, Sanders. = Pterygo-mandibularis, Bradley, Watkinson. == Pterygoideus, Versluys. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V3) Capiti-mandibularis.—This is the large temporal muscle which is divided into three slips in this paper. (a) Capiti-mandtbularis superficialts (C. m. s.).—This masseter slip is lower down on the outside of the mass. It arises on the quadrate, squa- mosal and quadratojugal. Its fibers extend over the fibers of the temporal slip and blend with them to be inserted on the coronoid and the dorsal edge of the mandible. They extend down on the outer face to quite an extent, which is not common in the reptiles, as usually the insertion is entirely on the inner side of the mandible. They also extend down on the inner side. This crossing of the temporal and the masseter slips gives two pulls to the mandible, a straight dorsal and a posterior one. (b) Capiti-mandibularis medius (C.m. m.).—The temporal or median slip fills the temporal fossa; it arises on the parietal, postfrontal and squamosal. The supratemporal fossa is large and the parietal is extended out, so as to furnish a large insertion for this part of the muscle. It ex- tends down between the masseter and the inner slip to be inserted on the mandible. : ADAMS, PHYLOGENY OF THE JAW MUSCLES 97 (c) Capiti-mandibularts profundus (C. m. p.).—This muscle is the homologue of the muscle of the same name in Varanus and is strongly developed in reptiles with a columella cranii (epipterygoid). It arises on the outer face of the epipterygoid-and on the outer face of the ptery- goid, extends ventrally, and is inserted on the mandible on the lower part of the posterior inner face of the mandible. Pterygoideus anterior (Pt. a.).—This arises on the pterygoid and wraps around the end of the mandible. The origin is on the under side of the pterygoid; from there it extends around the end of the mandible, making a large belly at the posterior ventral end. It is inserted along the ventral face of the angular and wraps around the articular, so that it is on the inside of the mandible at the posterior end. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (D. m.).—This muscle arises on the posterior face of the parietal, extends ventrally, and is inserted on the retroar- ticular process of the mandible. VARANUS Plate VII, Figs. 3, 4, 5 The almost snake-like skull of Varanus is a typical example of the streptostylic skull, with the bones of the skull more or less movable, espe- cially in the quadrate region. ‘There is a joint between the frontals and the parietals which has possibilities of movement, while the quadrate with its loose articulation on the squamosal and pterygoid forms another movable element. In fact the jaws, pterygo-palatine, and the quadrate region are all movable, with the posterior part of the skull as a base. The fenestre are large so that the muscles have room for development. The large supratemporal fenestra serves as the origin of the temporal slip of the capiti-mandibularis, while the lateral fenestra gives it room for development. This fenestration approaches the ophidian type where the opening out has been complete. The musculature of the pterygoid region is very musch specialized in the streptostylic forms and is a cenotelic character. These special muscles could not have been present in the cotylosaurs or stegocephalians, where there is no need of them, as the skull is rigid. The special muscles for moving the upper jaw and the facial part of the skull arise on the parietals, epipterygoid and basisphenoid and are attached along the whole upper surface of the pterygoid from the articulation with the transverse 98 ANNALS NEW YORK ACADEMY OF SCIENCES to the quadrate. This type of musculature is carried to the extreme in the Ophidia, where the streptostylism is complete and practically all of the bones are movable. There is a sharp contrast between the strepto- stylic and the monimostylic types of musculature ; the streptostylic repre- sents high specialization, with great mobility of the skull parts and the demand for many muscles, while the solid monimostylic skull is rigid with the bones solidly fixed and attached by strong sutures, while the muscles present are of a simpler kind, namely, those needed to raise and lower the mandible. There is much confusion in the nomenclature of the muscles owing to the fact that the mammal names are applied to them by most workers. The nomenclature would be cleared if the entire lst of names were dropped and a new set adopted that had no direct reference to the names of the mammalian muscles.* MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y,) Capiti-mandibularis. (a) Capiti-mandibularis superficialis (C. m. s.). = Outer fibers of the capiti-mandibularis. (>) Capiti-mandibularis medius (C. m. m.). = Temporal of Bradley (whole mass). (c) Capiti-mandibularis profundus (C. m. p.). —= Upper shp of the pterygoid of Bradley. = Pterygoideus externus of Mivart. Pterygoideus anterior (Pt. a.). = Pterygo-mandibularis of Bradley. = Pterygoideus internus of others. Pterygoideus posterior (Pt. p.). —= Lower slip of the Pterygoideus of Bradley. Pterygo-parietalis (Pt. par.) (Bradley). Pterygo-sphenoidalis posterior (Pt. sph. po.) (Bradley). MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VIL) Depressor mandibule (D. m.). MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Capiti-mandibularis.—This is the large temporal muscle mass, divided into three parts in this paper. 4Phe papers used in the study of Varanus were: Bradley (1903) and Watkinson (1906). ADAMS, PHYLOGENY OF THE JAW MUSCLES 99 (a) Capiti-mandibularis superficialis (C. m. s.) == Masseter portion. Arises on the quadratojugal, squamosal and quadrate. (b) Capiti-mandibularis medius (C. m. m.)—This is the inner part of the temporal sheet arising on postfrontal, prodtic, squamosal and quad- rate under the fibers of the outer or masseteric portion. These two slips are not separate. (c) Capiti-mandibularis profundus (C. m. p.).—The deeper portion of the mass. It arises on the parietal and is inserted on the coronoid along with the rest of the capiti-mandibularis. (A lower part of this muscle arrses from the columella cranii and is considered as pterygoideus posterior. ) Pterygoideus anterior (Pt. a.)—This muscle arises on the whole outer surface of the pterygoid (except the branch to the palatine) and is in- serted on the posterior part of the jaw. Pteryqoideus posterior (Pt. p.).—This with the exception of the lowest part arises on the columella cranii and is inserted on the mandible, posterior to the insertion of the capiti-mandibularis profundus. Pterygo-parvetalis (Pt. par.).—This muscle is peculiar to reptiles with a rodlike columella cranii. It arises on the downward projection of the parietal and is inserted on the whole upper surface of the pterygoid, im- mediately in front of the articulation with the columella. It seems to represent a part of the inner layer of capiti-mandibularis profundus that separated off for the service of the pterygoid. The same might be said of the muscle following. Pteryqo-sphenoidalis posterior—This muscle arises on the basisphe- noid below the gasserian notch and is inserted on the upper and lower surface of the pterygoid for its whole length to a level of the articulation of the basipterygoid process of the sphenoid and the pterygoid. It pulls the pterygoid upward and backward. This may represent a lower slip from the capiti-mandibularis profundus. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (D. m.).—It arises on the parietal and nuchal fascia and is inserted in the end of the mandible. AVES GALLUS Plate VIII, Figs. 1, 2, 3 The musculature of the bird, Gallus, follows in most particulars that of reptiles. It offers one of the most convincing items of evidence that 100 ANNALS NEW YORK ACADEMY OF SCIENCES the birds and the reptiles are very close in their origin. The most im- portant characters of the bird skull are: (1) Great expansion of the brain case with corresponding changes in the base of the cranium. (2) Loss of the upper temporal arcade, the lower temporal arcade (quadratojugal and jugal) being left intact. (3) Radical changes in the pterygoid and palatine region where great changes occur even within the class. (4) Loss of teeth and assumption of a beak. (5) Loss of sutures and lightening of all the elements. The bird skull is fundamentally similar to that of the primitive pseudosuchian Hupar- keria capensis of Broom (1913). The maxille, jugal and quadratojugal articulate with the quadrate in true reptilian fashion, while the quadrate is movable as in many reptiles. ; MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y,) Capiti-mandibularis. (a) Capiti-mandibularis superficialis (C. m. s.). = Masseter of Shufeldt and Gadow. (>) Capiti-mandibularis medius (C. m. m.). = Temporalis of Shufeldt, Gadow. (c) Capiti-mandibularis profundus (C. m. p.). Not present in Gallus. Pterygoideus anterior (Pt. a.). Pterygoideus posterior (Pt. p.). MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) The capiti-mandibularis mass of the bird is divided into separate muscles, and we assume that they are represented in the reptiles as un- separated parts of the capiti-mandibularis, and for that reason they are given the names used in the reptiles. (a) Capiti-mandibularis superficialis (C. m. s.).—This muscle may be divided into two parts; one rises as a tendon from the bony ridge of the squamosal, above the auditory entrance ; the other head which is fleshy arises from the quadrate. The insertion on the mandible is by ADAMS, PHYLOGENY OF THE JAW MUSCLES 101 tendon and by a fleshy slip; the tendon is inserted on the coronoid process and the fleshy slp extends along the outside of the mandible. This muscle is the most important of the mandible, as it extends along the side and has a very wide attachment. (b) Capiti-mandibularis medius (et profundus) (C. m. et p.).—This muscle, usually called the temporal, is assumed to represent the middle and deep part of the reptilian capiti-mandibularis. The region for its origin is much reduced and there is no room for much differentiation of this part of the muscle. It fills the temporal fossa in the bird. It arises from the bones surrounding the fossa, extends downward, posterior to the postorbital process, and is inserted on the coronoid process of the mandible. Pterygoideus anterior (Pt. a.).—This is a large muscle that covers the floor of the skull at the posterior part of the palatines. It arises along this posterior part of the palatines, spreading out over this to a part of the pterygoids and the orbito-sphenoid. The insertion is on the under side of the articular process and along the inner side of the mandible. This muscle pulls inward and forward on the mandible, thus pulling the quadrate forward—a movement that is very important in the Psittaci where the maxillz are raised with the anterior part of the skull. Pterygoideus posterior (Pt. p.).—This is a smaller muscle that crosses the pterygoideus anterior, going under it. It arises on the orbital process of the quadrate and is inserted on the inner side of the mandible near the posterior end. It draws the mandible backward and pulls down on the quadrate, thus aiding in the closing of the jaw. >) MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (1). m.).—The depressor of birds is the same as that of reptiles and amphibians, being a part of the C,md of Ruge and innervated by the seventh nerve. In the birds it is short and thick, more like the same muscle in the Chelonia than hke that of Sphenodon or Varanus. It arises on the squamosal as in the Chelonia, as this region has grown out laterally, so that the origin on the parietal is no longer available. It arises along the ridge of the squamosal and is inserted on the retroarticular process as in reptiles. It opens the mandibles with the aid of some of the long, ventral hyoid muscles of the throat region. 102 ANNALS NEW YORK ACADEMY OF SCIENCES MAMMALIA The jaw muscles of the mammals are remarkably constant in character throughout the group, except in the monotremes and some edentates where extraordinary conditions occur. Throughout the group the muscles are closely correlated with the dentition and with function. The chewing muscles are limited to the digastric, masseter, temporal, pterygoideus externus and pterygoideus internus. There is much individual variation in these muscles, but they are always present, except the digastric, which is absent in some monotremes and edentates, and the internal pterygoid, which is sometimes not differentiated from the temporal. The temporal fosse of the mammals are opened out, so that there is free play for the muscles, except where there is a secondary covering of plates. The ramus mandibularis of the trigeminus nerve innervates all the muscles of mastication except the posterior belly of the digastric and its allies. The muscles innervated by V, represent what remains of the capiti-mandibularis of the reptiles. The temporal is the largest muscle of this group in most of the mammals. It is closely associated with the masseter, which is a slip that has separated completely in the mammals, but still remains a part of the temporal mass in the reptiles. The tem- poral is variable in its size, as the masseter is often larger, usurping its functions in the rodents and in some other groups. It is partially divided in the mammals, as we find it with a single head in Homo and with three in the monotremes, but although the condition may vary the origin and insertion remain quite constant. The masseter is much more variable than the temporal muscle. It is double-headed in man, and also is divided into two muscles in the mono- tremes, each with two slips, and we may have as many as five slips in the rodents. Its size and condition depend on the special conditions and the importance of the muscle. In the rodents, where it takes the place of the temporal to a certain extent, it is large and very much specialized. Its origin varies, as it may partly cover the temporal as in Didelphys, or may extend forward, anterior to the eyes in the rodents, or divide into two separate muscles as in the monotremes. This muscle in the mammals is always associated with the development of the zygomatic arch and repre- sents the outer portion of the temporal mass that may be traced from the Pisces to reptiles, where the outer fibers of the capiti-mandibularis mass are always slightly differentiated from the deep fibers by having a dilfer- ent direction, although there is no separation in the reptiles to the extent found in mammals. The pull of the masseter is usually slightly forward, = ADAMS, PHYLOGENY OF THE JAW MUSCLES 103 against the pull of the temporal, which, together with the pull of the pterygoid muscles, gives the steadiness that is necessary in the mandibles. In many cases there is a connection of the masseter with the auditory region. In Tamandua (Lubosch, 1908, p. 541) this small muscle is seen, as it were, in the process of splitting off from the masseter and attaching itself to the tympanic. Bradypus villosus (Luubosch) shows this same condition, where a small muscle extends from the angle of the jaw to the tympanic. This small muscle is innervated, according to Lubosch, by the auriculo-temporalis nerve. The tendency of the masseter to extend to the auditory region probably may go back to the reptilian condition, where this muscle was attached to the quadrate or to one of the bones in this region. The pterygoid muscles arise on the basicranial or pterygoid region and, although usually small, they are of importance in the working of the jaws. They vary in form in different groups and the relation between the pterygoids and the dentition is very close. They are largely devel- oped in herbivorous but small in carnivorous mammals. They function in giving the jaw steadiness in opening and closing; they prevent the heavy temporal muscles from tearing the mandibles apart in forms with a loose symphysis and they make possible the grinding motion of the teeth in herbivorous forms. The edentates and monotremes offer excep- tional conditions, for here the reduced or modified condition of the man- dible makes the pterygoid muscle of little importance. The internal pterygoid is the more important of the two, as it aids in the closing of the jaw and pulls inward against the pull of the temporal, masseter, etc. The external pterygoid pulls the jaw forward, out of the glenoid cavity, and assists in this way in opening the jaw and in the forward movement of the jaw in herbivorous forms. The origins of these muscles are plainly seen in mammalian skulls and the condition of the pterygoid region has probably been very much modi- fied by their action, as a comparison with the reptilian condition shows that the pterygoid region is pinched in and pulled to the rear. We assume that this condition has been brought about largely by the action of ptery- goid muscles. In cynodonts the opposite pterygoid bones meet in the mid line, but in mammals the pterygoids have become much reduced and separated so as to leave the primary floor of the brain case exposed (basi- sphenoid, presphenoid) (Gregory and Adams, 1915). The capiti-mandibularis profundus (external pterygoid) of the reptiles seems to be in a favorable position to give rise to the whole pterygoid mass of the mammals, since it is inserted on the coronoid region of the mandible, and might easily shift its insertion down on the inner side of 104 ANNALS NEW YORK ACADEMY OF SCIENCES the upgrowing dentary without interfering with the other muscles. No doubt its importance increased when the new joint was formed between the dentary and the squamosal and when the old anterior pterygoid muscle became reduced. Its subdivision into two slips followed, which are the external and internal pterygoid of mammals. The digastric muscle of mammals (Fig. 4) represents a part of the second constrictor of the Pisces, joined with one of the ventral muscles of the throat region (A, V,). It is a muscle with two bellies, a tendon usually separating them, but this varies in different forms; so we may say that the muscle varies from the so-called monogastric to the typical double-bellied condition. As stated below, the muscle is constant in mam- mals with the exception of monotremes and some edentates. It is the only compound muscle in the muscles of the jaw and represents two muscles, one innervated by the facialis, the other by the ramus mandibu- laris trigemini, joined end to end, but still retaining the old innervation. The older anatomists all homologized the posterior belly of the digastric with the depressor mandibule of reptiles and amphibians, but the work done on the innervation by Schulman, Lubosch, Ruge and others has shown that there is a common origin for them, but that they represent different slips from the same constrictor. The variation of the digastric has been discussed by Chaine, Toldt, Bijvoet, Parsons, Rouviére, Fiirbringer, Dobson, Futamura and others, so that there is not much left to work out in this line, although the inter- pretations of the authors are very variable. Chaine (1914) classifies the digastric of mammals as follows: (a) Those with two bellies: Macropus Bradypus Delphinus (Toldt) Artiodactyla Rodentia Carnivora Chiroptera Pteropus Insectivora ° Simi Prosimize (b) Those with a single belly: Hydrocherus Cavia Dolichotes Lepus (c) Digastric absent: Monotremes Delphinus Tatusia Tursiops 5In Talpa europawus there is a small tendon from the posterior belly to the mandible. ADAMS, PHYLOGENY OF THE JAW MUSCLES . 105 Parsons’ views (1898, pp. 436-437) are expressed in the following quotations : The most important point to bear in mind, in considering this muscie, is that it is not always really a digastric, but that even when it appears monogastric it has a double nerve supply. Our knowledge of the anatomy of fishes tells us that the muscles developed from the first, or mandibular arch, are supplied by the fifth nerve, while those formed in the second, or hyoid arch, derive their nerves from the seventh. In three Ornithorhynchi I found a muscle running from the sub-hyoid septum, outward and a little forward to the man- dible, not far from the angle; it was supplied by the fifth nerve and lay super- ficial to the mylohyoid muscle, of which it appeared to be a delamination, and with which the fibers had the same general direction. In the same animal a single muscle runs from the long, tubular external auditory meatus to the sub- hyoidean septum; it is supplied by the facial nerve, and probably corresponds to the combined stylohyoid and posterior belly of the digastric. From this I am inclined to think that the anterior belly is an older muscle than the pos- terior, or, in other words, that the anterior belly is differentiated from the mylohyoid layer before the posterior belly is split off from the stylohyoid. This possibility may be the reason why the absence of the anterior belly of the digastric is rare in man, but the absence of the stylohyoid is fairly common. Describing the digastric with two belles, Parsons says: In the first [type] the anterior and the posterior bellies are separated by a considerable length of tendon. The posterior belly runs forward until it is over the hyoid and then runs inward and meets its fellow from the opposite side, forming an arch. The anterior bellies of the digastric spring from this arch and go forward. The muscle is not attached to the hyoid but is connected by connective tissue. This type is found in many rodents (sciuromorphs, Pteromys), in most cynomorph monkeys and in certain anthropoid apes (orangs, chim- panzee). Parsons places under his second division those with a pseudo-mono- gastric muscle. Here the muscle seems to have one belly, but there is always a small rudiment of the median tendon. This type attaches to the mandible, midway between the symphysis and the angle, and instead of forming flat planes meeting along the median line of the neck they are rounded, as some expanse of mylohyoid separates them. Parsons’ third type is that found in man, where the bellies are distinct with a tendon separating them, the anterior bellies not meeting in the median line of the neck but forward near the symphysis. This type is found in most lemurs, many monkeys, especially the platyrhini, gibbons (Hylobates) and in many specimens of gorilla. The stages are all united by transitional stages. ADAMS, PHYLOGENY OF THE JAW MUSCLES 107 Parsons also mentions a fourth type, where there is a true monogastric muscle in which the anterior or the posterior belly is suppressed. It is found in lagomorph, or hare-like rodents. The posterior belly is repre- sented by a small tendon in these. In many orangs the anterior belly is wanting and the posterior belly is attached close to the angle. Parsons’ division differs from Chaine’s in that he takes into considera- tion the condition of the anterior belly, whether joined with its fellow of the opposite side or free from it. Humphrey considered that the anterior belly was from the same myotome as the pterygoids, and the posterior belly was from the hyoid arch muscles, or hyoid myotome. He considered the tendon as a remnant of the myocommata connecting the two myotomes. These divisions of the digastric as given by Chaine and Parsons show some of the variations that are met with in this muscle. The posterior part of the muscle is not troublesome, but the anterior part, or the part innervated by nerve V,, has caused the anatomists much trouble. Some of the ideas as to the homology of the anterior belly are interest- ing, and show the diversity of opinion on this topic. Bijvoet (1908) gives an interesting summary of the ideas of different authors as to its origin. He considers that Ornithorhynchus shows the primitive stage of the DESCRIPTION OF FIGURE 2 Variations of the digastric in mammals, (Mainly after Chaine and Parsons.) The digastric muscle is found only in typical mammals and is not clearly recognizable in the monotremes. It is a compound muscle typically consisting of anterior and pos- terior bellies united by ligament. The anterior belly is probably a derivative of the primitive throat muscles of reptiles and is innervated by the mylohyoid branch of nerve V,. he posterior belly probably represents a separate slip from the stylohyoid muscle, and both muscles are innervated by closely associated branches of the seventh nerve. Probably both bellies of the digastric formerly converged toward the lower surface of the basi-hyal cartilages, along with the mylohoid and other muscles, and the tendinous portion between the anterior and posterior bellies may represent part of the fascia into which they were formerly inserted. Probably the most primitive type is seen in the monotreme Ornithorhynchus (10). Here the “detrahens mandibulew anterior’ (D. m. a.) (which may be a slip of the mylo- hyoid) may represent the anterior belly of the digastric, while the posterior belly is not yet separate from the stylohyoid (Parsons). The marsupial Macropus (5) shows well the association of the anterior belly (a) with the mylohyoid and the connection of both bellies with the basi-hyal. In most types the anterior and posterior bellies are both present and separated by tendon (as in Figs. 2, 4, 5, 6, 8, 9, 11). Hither one of the bellies may be vestigial or wanting. In No, 7 the posterior belly is reduced to a small tendon. In No. 13 the anterior belly is tendi- nous. The so-called monogastric types (3, 6, 9) generally show at least a vestige of the tendinous septum between the two bellies. In Nos. 1, 12 the digastric is entirely absent, being functionally replaced by other muscles. The posterior belly usually arises from the paroccipital process of the exoccipital. The insertion of the anterior belly varies greatly as well as its relations both to its fellow of the opposite side and to the mylo- hyoid (cf. Nos. 2, 3, 7, 9, 11). It is primitively inserted on the inferior border of the mandible beneath the masseter, but may shift either to the region of the symphysis (2) or to the posterior part. . 108 ANNALS NEW YORK ACADEMY OF SCIENCES formation of the digastric and that the anterior belly is the detrahens mandibule, a muscle related to the stylohyoideus. Chaine (1914) thought that the digastric came from a muscle that originally stretched from the jaw to the sternum, that it lost its hold on the sternum and moved first to the vertebre and then to its hold on the mastoid. He accounted for the tendon between the bellies of the digastric by assuming that it was a remnant of the segmental areas found in primitive muscles. Dobson does not consider the digastric. Futamura (1906, 1907) had some interesting ideas on the origin of the digastric of man. In the first paper on the digastric of man he says: “The digastric is at first entirely supplied by the nervus facialis; later, as the anterior belly becomes constricted off from the posterior, the former obtains its motor nerve secondarily from the nervus mylohyoideus” (translation). He recants this in a later paper (1907) as follows: Der proximal Teil des Digastricus teilt sich in zwei Teile deren yorderer am Reichertschen Knorpel inseriert, deren hinteren um die hintere Seite des Knorpels herum ventralwirts verliuft und am ventralen Ende des Meckelschen IKxnorpels inseriert. Der Muskel wird von zwei Nerven bereits innerviert; N. facialis und N. mylohyoideus. Beim Menchen glaubte ich aussprechen zu diirfen dass die zwischensehne an der doppelten Innervierung des Muskels schuld sei. Das kann aber doch nicht der Fall sein, weil beim Schwein, bei dem der Biventer keine zwischensehne besitzt, doch die zweifache Innervierung nachzuweisen ist. Gegenbaur (1898, p. 632) held that the anterior belly of the digastric came from the mylohyoid by splitting. He took the anterior belly from the mylohyoid and the posterior belly from the depressor mandibule of the reptiles. He cited the horse with its peculiar condition, where there is a secondary insertion of the posterior belly on the angle of the mandi- ble, as an indication of this. His (1885, p. 92) derived the digastric from the sterno-cleido-mastoid, which he separates into two parts: the mylohyoideus and the outer tongue muscles. He derives the anterior belly of the digastric from the super- ficial layer and the posterior belly from the deep layer of the sterno- cleido-mastoid. Leche (1889) had the same idea as Futamura and thought that the digastric was a muscle with a single nerve and that it acquired, second- arily, a second supply from the trigeminus. Rouviére (1906) derived the anterior belly from the same origin as the geniohyoid and says that in the fishes, amphibians, reptiles and birds, only the anterior belly is present. He derives the posterior belly from ADAMS, PHYLOGENY OF THE JAW MUSCLES 109 the m. jugularis transversus, the same mass from which the stylohyoid is separated. He considers the digastric to be the result of the joining of these two muscles. Toldt (1908) gives the following points on the anterior belly of the digastric : (1) The digastric as such is found only in the mammals. The attempts to derive it from the muscles of the non-mammalia have not been success- ful. The hinder belly is not the depressor mandibule of the Sauropsida nor is it derived from the depressor of the monotremes. (Schulman shows that it is not derived from the depressor of monotremes. ) (2) The depressor of the monotremes is a special primitive condition and is a slip from the masseter innervated by V,. The writer thinks this is not a primitive condition but a very specialized condition in a primitive form. (3) The anterior belly of the digastric has its origin with the m. mylohyoideus muscle and belongs with the visceral muscles that extend along the floor of the mouth. They are innervated by V,. (4) The posterior belly is a part of the stylohyoideus in the lower vertebrates and is originally a visceral muscle intercalated between the musculature of the mouth floor and throat and directly or indirectly con- nected with the tongue bones. (5) The joining of this originally separate muscle to the digastric, as well as the identity of the hinder belly and the stylohyoideus, is shown well in the monotremes. The line of separation is shown by the inscriptio tendinea. Toldt correlates the condition of the digastric with the type of food that the animal eats and with the question whether the food is held in the mouth or swallowed immediately. This, he thinks, gives an explana- tion of the joining of the forward bellies in some forms and separation of them in some of the carnivorous forms. But a comparison of the varying forms of digastric seems to show that there is no correlation with food habits, as there seems to be no definite functional criteria by which the different conditions may be classified. From a review of the literature on the digastric the variations are plainly evident both in the muscle and in the ideas concerning it. There seems to be no very stable insertion for the anterior belly. Its insertion varies from the inside of the chin, as in Homo, to the posterior part of the mandible, on the angle in some other forms. Only the anterior part is of uncertain origin, as most of the writers agree on the derivation of the posterior belly. The anterior part lends itself to hypotheses, as nothing definite seems to have been brought forward at the present time 110 ANNALS NEW YORK ACADEMY OF SCIENCES and all may be more or less right. The double innervation and the tendon seem to make certain the fact that the muscle has a double origin. Some of the older writers speak of a single-bellied digastric, but most of them agree that some evidence of the double origin always exists, and that in the monogastric muscles remains of the tendon are in evidence in the middle of the muscle, or, as in Lepus, the anterior muscle may be almost gone while the tendon remains to represent the junction of the two parts. In a few forms undoubtedly one of the muscles is missing— that is, one belly is missing and it is not really a monogastric muscle but a single belly in the sense that either the anterior or the posterior belly has disappeared, and not that the one represents both bellies. So far I have spent very little time on the mylohyoid and the other ventral muscles, reserving them for future work on the comparative musculature throughout the vertebrates. The discussion of this phase of the jaw muscles is made very brief and merely points out some of the comprehensive work of Toldt, Chaine, Rouviére, Bijvoet and others on the digastric musculature in the mammals. The relation of the jaw muscles to the special types of dentition is very close throughout the mammalian groups. In fact a highly differentiated muscle system in which each muscle does a certain type of work alone makes possible the highly specialized dentitions of rodents and ungulates. The teeth of mammals are, roughly, of three kinds—herbivorous, carniv- orous and degenerate. The herbivorous dentition is correlated with the peculiar type of condyle and glenoid joint that makes its specialized work possible. The condyle of the typical herbivorous animal is much rounded and the articulation in the glenoid cavity is flat, so that there is much freedom of movement. In chewing the jaw is rotated in a lateral, antero- posterior and vertical movement, so that to accommodate this movement the articulation must be very loose. Accordingly, there is always a well developed pterygoid region in the basicranial region, with well developed pterygoid muscles. This strong development of the pterygoid muscles is characteristic of Macropus, Halmaturus, Castor, Sciurus, Equus and Bos. In these animals the molar teeth bear cross-ridges and the transverse mo- tion needed for trituration is given by the strong pterygoid muscles. Many herbivorous mammals chew on one side at a time. The pterygoid muscle, acting in connection with the large temporal and masseter muscles of one side, make this movement possible. In the rodents where the symphysis is weak the pterygoid muscles also counterbalance the pull of the huge masseters, as in some of the forms with a weak symphysis the pull of the masseters alone would tear the jaws apart. The carnivorous mammals have an entirely different ar- ADAMS, PHYLOGENY OF THE JAW MUSCLES Halal rangement, as their jaw action is primarily a straight opening and closing movement, either a hard, steady pull, or with a snap. The opposite halves of the mandible are usually strongly fastened together at the symphysis. The condyle is lengthened out laterally toward the median line and the glenoid cavity has a large protecting shelf to give a firm hold on the condyle. In fact in some of the carnivorous types with strong jaws the articulation is locked, so that the borders of the glenoid cavity must be broken in order to separate the mandible from the skull (Gulo luscus). With the shearing teeth and the locked type of articulation for the con- dyles of the jaw the pterygoid muscles are of little value and in carniv- orous forms are always weak, as in Canis, Gulo, Lutra, Didelphys. In these animals the basicranial region of the skull shows the reduction of the pterygoid muscles. MONOTREMES Plate IX, Figs. 1-7 Skull.—The skull and jaws in both Ornithorhynchus and Echidna are of very aberrant and more or less degenerate types. In both genera, also, the pterygoid region is highly modified, and thus the homology of the so-called pterygoid elements is open to question. Ornithorynchus re- quires fairly strong jaws for the crushing of small mollusc shells, while Echidna has practically vestigial jaws and depends mostly on the tongue muscles. The condition of the jaw musculature in the monotremes is quite sug- gestive of the reptiles. The massed condition of the temporal-masseter group suggest the condition of the capiti-mandibularis of the reptiles. The head muscles as a whole seem to be homologous with those of other mammals, especially as regards the temporalis-masseter, pterygoideus externus and muscles of the ventral hyoid group, with the exception of the anterior belly of the missing digastric, which may be represented by the muscle called depressor mandibulx anterior. However, this hyoid region in the monotremes is very specialized, so that the derivation of the an- terior belly from this region is rather obscure, although some anatomists hold this opinion. It is probably a slip from the mylohyoid. The duck- like bill of the Ornithorhynchus requires a special musculature which is developed from the V, muscles. The monotremes are so different from other animals in many important details of their myology that it is probably correct to assume that they split off from the mammalian stem at a very early period. This assump- tion is strengthened by the fact that the osteology and soft anatomy are 112 ANNALS NEW YORK ACADEMY OF SCIENCES ~ also very aberrant, so that it is not strange to find different conditions in the musculature. The jaw muscles of monotremes include the following: MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YJ,) Masseter Pterygo-tympanicus Temporalis Depressor mandibule anterior Pterygo-spinosus Detrahens mandibulee MUSCLES OF THE DIGASTRIC GROUP (INNERVATED BY VII) These are not present. Monotremes have the following peculiar conditions of the jaw muscles: They have no true digastric. The masseter and temporal muscles are massed together. They have no pterygoideus internus. Presence of the pterygo-spinosus and pterygo-tympanicus. Presence of the detrahens mandibule instead of the digastric. Extensive development of the mylo- hyoid group of muscles. Absence of the digastric—The digastric is fairly constant in the mam- mals except monotremes, edentates and some Cetacea. Perhaps it is ab- sent in the monotremes because they separated from the mammal stem at a very early period and thus have missed this arrangement. Ornitho- rhynchus with its fairly large jaws needs a depressor, and this function is filled by the detrahens muscle and by the depressor mandibule anterior of the hyoid group. Hchidna is so degenerate in the mandibular region that there is very little need for a specialized depressor. Other mammals in which this muscle is missing have degenerate jaws. Chaine mentions Tatusia of the edentates and Delphinus of the Cetacea, where the true digastric has been lost. Imperfect separation of the masseter and temporal.—The condition of the masseter-temporal mass in the monotremes is very suggestive of the capiti-mandibularis in the reptiles, as it is not differentiated as much as in other mammals. The mass has several heads, but the separation is not so complete. The peculiar shape of the skulls in the monotreme may be responsible for this condition. There is very little room for muscles in Echidna, as the mass is completely covered by bone. Schulman (1906) gives a division of the muscles showing their complexity and tendency to break up into small slips. Absence of the internal pterygoid.—The absence of the pterygoideus internus is striking, as it is one of the constant muscles of the mam- malian skull. Some writers gave the monotremes an internal pterygoid, t ADAMS, PHYLOGENY OF THE JAW MUSCLES 113 but from the recent work of Schulman (1906) it seems that this is a mistake. Evidently its separation from the temporal mass is not com- plete. Schulman says that the pterygoideus internus of Meckel is inner- vated by a nerve that supplies the temporalis and assumes that this muscle is a part of the temporalis, which he calls “caput anterius.” He offers the suggestion that it might have appeared in the mammals after the monotremes had been cut off from the mammalian line. He says: Der M. pterygoideus internus wird, aller Wahrscheinlichkeit nach, bei Ornithorhynchus vermisst. Nicht einmal das Mikroskop zeigte in der ventral und medial von dem R. III trigemini befindlichen Gegend Muskelreste zwischen dem Schiidel und dem Unterkiefer, d. h. an dem Platz, wo dieser muskel bei den Siiugethiere vorzukommen pflegt. Ob die Wesenheit des M. pterygoideus internus bei den Monotremen ein Riickbildungserscheinung ist, oder ob dieser Muskel erst mit dem Typus der tiber den Monotremen stehenden echten Siiugethiere enstanden ist, bleibt vorliiufig eine offene Frage. The homology of the “pterygoid” in monotremes is in question, and if the conclusions of some of the authors are true, the small muscle called the pterygo-spinosus might be called a pterygoid muscle but for the fact that it appears in man as a rudiment or an anomaly and appears as a regular element in Cholepus, Tamandua, Manis, Tatusia and Dasypus. Presence of the pterygo-spinosus and pterygo-tympanicus.—The pres- ence of the pterygo-spinosus and the pterygo-tympanic suggests the un- settled state of the pterygoids in mammals, for in the animals in which it is found the attachment greatly varies. Its positions are as follows: (a) It is attached to the sphenoid in man and to the lamina lateralis of the pterygoid process (Thane, McAllister, Poland, Kreutzer). (b) It is attached to the ligament assessorium mediale (Kreutzer). (c) It is attached to the mandible (Gruber, Kreutzer). (d) It is attached to the pterygoid internus (Poland, Gruber, Kreut- zer). From these variations Lubosch believes that the insertion is lost. Schulman (1906) says: Es kann daran gedacht werden, die bei den héheren Siiugethieren vorkom- men zwei Muskeln auf das Velum palatinum sich ausbreitenden M. tensor veli tympani und den an einem unbeweglichem Pterygoidknochen sich anheften rudimentiiren M. pterygo-spinosus, vor einen, bei den Promammalia mit einem beweglichen Pterygoideum verbunden, undifferenzirten Muskel abzuleiten, der danach as Muttermuskel fiir die M. tensor veli palatini und pterygo-spinosus anzusprechen wiire. Doch bedarf es zur Sicherung dieser Vermutung viel ausgebreiteterer Untersuchungen als die hier angestellten. Auch wire heirbei die Frage zu erértern, ob die Beweglichkeit des Pterygoids von Ornithorynchus eine primiire oder sekundire ist. 114 ANNALS NEW YORK ACADEMY OF SCIENCES The most typical attachment of this muscle is to the annulus of the ear and to the pterygoid bone from the mandible. In spite of all the work done on the pterygoids of mammals, one may say that they are of rather unsatisfactory status as yet. Gaupp assumes that they are derived from the parabasals and that they are not homologous with those of other mam- mals. It is hoped that the problem will be solved as a result of Watson’s recent studies on the development of the skull in the monotremes. Presence of the detrahens mandibule. This pair of muscles serves as the depressor of the mandible in monotremes where the digastric is en- tirely absent. It originates on the mastoid and squamosal region of the skull, wraps around the lower part of the head, and is inserted on the lower edge of the mandible with a good attachment on the sides, so that it has a firm hold. It is peculiar to the monotremes and is not homolo- gous with the parieto-mandibularis (depressor mandibule) of reptiles which has a similar position and function. The reptilian analogue is innervated by the seventh nerve, while the detrahens mandibule in mono- tremes is innervated by the trigeminus. Schulman, Toldt, Bijvot and Gaupp believe that it belongs to the dorsal muscles of the head. Gaupp and Schulman both believe that it is a slip of the capiti-mandibularis (masseter portion) of the reptiles that has slipped back to act as the depressor. ‘The earlier investigators thought that it was a part of the mammalian digastric, as they did not know of the nerve supply. All of these investigators have changed their opinion with the working out of the innervation by Schulman, who showed that it is innervated by the trigeminus nerve. Schulman says: Es unterliegt somit meiner meinung nach keinen zweifel, dass der M. de- trahens mandibule zu den dorsalen Kaumuskeln gehort, und keinen Bauche des M. digastricus mandibule der hoheren Mammalia homolog ist. The meaning of this musculature in the monotremes seems to be that the reptilian articulation of the mandible to the jaw was lost, as the Reichert Theory would assume, and that in the shifting of the muscles in monotremes the new depressor was developed from the capiti-mandib- ularis instead.of from the depressor mandibule or its mother mass. This would place the monotremes in a different line, and their anatomy justly places them at some distance from the rest of the mammals, so perhaps this conclusion is not far wrong. ADAMS, PHYLOGENY OF THE JAW MUSCLES 115 DIDELPHYS Plate X, Figs. 1, 2 The skull of Didelphys is remarkably similar to that of lower Eocene Carnivora in so far as it has a very narrow brain case, high sagittal crest, long heavy muzzle, stout zygomata and stout curved mandibles. The pterygoid bones are reduced, as is frequently the case in carnivorous mammals. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Masseter Pterygoideus internus Temporalis Digastricus (pars anterior) Pterygoideus externus MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior ) MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter—This is a very large and strong muscle, arising on the under side of the zygomatic arch and from the fascia extending up over the temporal muscle. The insertion is on the posterior, ventral part of the angle of the mandible. Temporalis—This is a large muscle filling the temporal fossa and ex- tending to the high sagittal crest. The insertion is on the coronoid process of the mandible. Pterygoideus externus.—Arises behind the pterygoideus internus on the alisphenoid and is inserted on the inner side of the mandible. Pterygoideus internus.—Arises on the pterygoid wing of the alisphe- noid and is inserted on the inner side of the angle of the mandible. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus posterior—This is quite a large muscle, arising on the mastoid and inserted on the inner side of the mandible, well forward. 116 ANNALS NEW YORK ACADEMY OF SCIENCES SOLENODON Plate X, Figs. 3, 4 The skull of Solenodon is remarkable for its degenerate zygomata, for the great elongation of the muzzle, verticality of the enlarged anterior incisor, small brain case with low sagittal crest, and sharply tritubercular molars. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter Pterygoideus internus ‘Temporalis Digastricus (pars anterior) Pterygoideus externus MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior) MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter—A thick, heavy muscle arising on the zygomatic process of the maxillary and from the fascia over the temporahs. It wraps around the posterior angle of the jaw. Temporalis.—A large muscle arising in the temporal fossa and on the squamosal. The insertion is on both the inner and outer surfaces of the coronoid process. Pterygoideus externus.——A small muscle with a small origin on the alisphenoid. Inserted on the inner side of the head of the condyle. The muscle is slight, as the animal has a carnivorous dentition. Pterygoideus internus.—Arises in the pterygoid fossa and is inserted on the inner side of the posterior angle of the mandible. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior ).—This is a large heavy muscle which arises on the mastoid process and is inserted on a bony process on the lower side of the mandible under the coronoid process. ADAMS, PHYLOGENY OF THE JAW MUSCLES at allre MUS Plate X, Figs. 5, 6 The relations of the jaw muscles to the dentition and form of jaw and skull are nowhere more apparent than in the rodents. The rodents have a great development and differentiation of the masse- ters which overshadows the temporals. This is characteristic of the group and the jaw action is very complex. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YJ,) Masseter Pterygoideus internus Temporalis Digastricus (pars anterior) Pterygoideus externus MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior ) MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter—This muscle is very large and much subdivided. It arises along the zygomatic arch, extending some distance in front of the eyes where the zygomatic process of the mandible is much expanded to give it a foundation. The posterior part of the expanded mandible forms the base of the insertion. This insertion is very large in comparison with that of most mammals. Temporalis.—This muscle arises in the temporal fossa and is inserted on the inner side of the mandible. It is of medium size in Mus, although in some of the rodents it is much reduced. Pterygoideus externus.—This muscle arises on the pterygoid fossa and is inserted on the mandible, where it has a good hold on the expanded angle. It holds against the pull of the masseters and pulls the mandible forward. Pterygoideus internus.—This muscle is much larger than the external pterygoid, arising in the pterygoid fossa and inserted with a large inser- tion on the large expanded angle of the mandible. It holds against the pull of the other temporal muscles and prevents the tearing apart of the jaws where the symphysis is weak, as it is in many rodents. 118 ANNALS NEW YORK ACADEMY OF SCIENCES MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior).—The digastric as a whole is a large muscle arising on the mastoid and inserted on the lower edge of the mandible, posterior to the symphysis. FELIS The cat furnishes a good example of a specialized carnivorous jaw with chiefly orthal or vertical movement. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter Pterygoideus internus Temporalis Digastricus (pars anterior) Pterygoideus externus MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior ) MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter.—This has three layers in the cat, but it has about the same insertion and origin as in man. Temporalis—This is more extended than in man, reaching to the parietal crest. Its insertion is on the outer side of the coronoid process. Pterygoideus externus.—Arises in the external pterygoid fossa between the foramen rotundum and the spheno-palatine foramen. Its insertion is on the inner side of the coronoid process. The coronoid process is very high and the condyle very low, so that the muscle has changed its inser- tion. Pterygoideus internus.—Arises in the pterygoid fossa in about the same position as in man. ‘The insertion is on the inner border of the mandible and extends back to the angle. Its fibers on the inner side go to the masseter, and it is partly united with the pterygoideus externus and the stylo-hyoid ligament. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior).—The digastric as a whole arises on the mastoid and occipital bones. Inserted on the lower edge of the mandible, just anterior to the first premolar. It is entirely inside of the jaw. ADAMS, PHYLOGENY OF THE JAW MUSCLES 119 HOMO Plate X, Figs. 7, 8 Riegner (1906) gives the results of some interesting experiments that he had made upon chloroformed apes. He stimulated the jaw muscles of the unconscious apes and watched the jaw actions, which may be summarized as follows: The masseter lifts the jaw and pulls it laterally. The temporalis lifts the opened jaw. The pterygoideus externus opens the closed jaw to some extent, pulling it slightly medianward and toward the opposite side. The pterygoideus internus closes or lifts the open jaw and pulls in opposition to the masseter, 1. e., toward the opposite side. The digastric (anterior belly) opens the jaw. Without it a nom- inal movement to the opposite side takes place. The hinder belly has little to do with the jaw movements in the ape. The geniohyoid gives the jaw a straight pull in opening it. This work on the apes gives about the action that takes place in man, but it is not an entirely accurate account of what happens in some of the mammals with a different type of skull (see also Strasser, 1908). MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter Pterygoideus internus Temporalis Digastricus (pars anterior) Pterygoideus externus ‘ MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Digastricus (pars posterior ) MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Masseter.—Arises from the lower border and inner surface of the zygomatic arch and is inserted on the outer face of the posterior end of the mandible and on the coronoid process. Temporalis.—Arises from the whole surface of the temporal fossa, ex- tends down under the zygomatic arch and attaches itself to a point of the coronoid process, the insertion extending slightly down on both sides. Pterygoideus externus.—Arises by two heads, from the under surface 120 ANNALS NEW YORK ACADEMY OF SCIENCES of the great wing of the sphenoid and from the outer surface of the ex- ternal pterygoid plate. It is inserted in a depression in front of the neck of the mandibular condyle and in the inter-articular fibro-cartilage and capsule of the temporo-maxillary articulation. Pterygoideus internus.—Arises by two heads: one from the deep sur- face of the external pterygoid plate and another by a stout tendon from the tuberosity of the mandible. Both heads are inserted in a triangular area on the inner, posterior surface of the mandible. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP Digastricus (pars posterior ).—The digastric as a whole arises in the digastric groove of the mastoid process of the temporal and is inserted in the lower, inner border of the anterior portion of the mandible. It is a two-bellied muscle, the two bellies being separated by a stout tendon. A slight tendon also attaches to the middle of the muscle to the hyoid bone. RECONSTRUCTIONS OF THE JAW MUSCLES IN CERTAIN IXTINCT VERTEBRATES PLACODERMI The great class of the Placodermi, including many specialized forms, apparently never attained true gnathostome jaws. Gregory says: Alt the known ostracoderms appear to be aberrantly specialized in certain directions, but long consideration of their many peculiar characters has con- rinced me that they stand far below the true fishes and that the group as a whole may represent an important stage in the genealogy of the vertebrates. In the upper Silurian we have many forms, hke Birkenia and Lasanius of the order Anaspida, Lanarkia and Thelodus of the order Heterostraci, without true gnathostome jaws, so far as has been discovered. The mouth in these forms was probably a sucking type with a sphincter around it. Dr. Gregory (1915) has the same idea: Of the many beautifully preserved specimens of Birkenia, Lanarkia, Dre- panispis, Pteraspis, Tremataspis, Cephalispis and allied genera, none show the least indications of an internal skeleton, nor is there ever any trace of the branchial arches and internal jaws. . . . The ostracoderms represent a stage in chordate phylogeny immediately preceding the acquisition of a carti- laginous skeleton impregnated with mineral salts; their first visceral arches, if present, had not been transformed into primary or cartilaginous jaws; the process of cephalogenesis was in a low stage and the elements of their shelly exoskeleton were potentially homologous with cosmine, vasodentine and isopo- dine of the primitive ganoids. To that extent they stand in a “pre-gnatho- i ADAMS, PHYLOGENY OF THE JAW MUSCLES 12] stome” stage of evolution and probably represent the forerunners of the ganoids and Tetrapoda, while possibly having remote relationships also in another direction with ancestral elasmobranchs. . . . The Antiarchi bave advanced beyond the typical Ostracoderms in having the head sharply differ- entiated from the thorax and the mouth armed with functional jaws, which are fashioned from the dermal skeleton. But not even the exquisitely pre- served specimens of Bothriolepis described by Patten (1912) show any traces of the cartilaginous jaws, branchial arches or cartilaginous axial skeleton. In another passage this author writes: The upper Silurian Birkenia of Traquair apparently had no biting jaws and may have sucked in small particles of food like the larval lamprey. Well preserved material showed that none of the Ostracoderms had cartilaginous jaws or teeth, but the dermal plaques around the oral hood sometimes fune- tioned as jaws. Typically carnivorous habits, involving true cartilage jaws, true teeth, and both paired and median fins, are first shown in the Acanthodian sharks of the upper Silurian and Devonian. We may therefore assume that cartilaginous jaws first appeared as such in the true fishes (elasmobranchs). In the Silurian and Devonian ostra- coderms there is a progressive series that shows some of the many attempts to produce workable jaws. The Anaspida, though fish-like in form, have progressed but slightly toward the true fishes. Lasanius had some struc- tures back of the poorly formed head that suggest dermal gill supports. In certain Heterostaci (Thelodus and Lanarkia) the skin was covered with denticles that resemble those of elasmobranchs. These denticles would strengthen the skin and give the muscles of the skin fascia much better support. The Drepanaspide show marked progress in the strengthening of the skin by the formation of plates of different sizes. Drepanaspis and Pteraspis show a differentation in the head region, but it is more to be compared to the hard covering of some insects, as the cartilaginous sup- ports of the interior were feeble or lacking. The mouth was unlike that of either elasmobranchs or teleosts, but seems to have been a wide slit stiffened by dermal scutes. The Ostrastraci show more specialization along this same line, but with no better results. The clumsy plates of Cephalaspis, Tremataspis, etc., give little promise of anything that would be much better than the arthropod jaw. The placoderm fishes of the order Antiarchi made a more clearly defined attempt to have movable jaws formed from dermal plates. A study of the fossils shows that they had a mouth that was functionally analogous to the mouth of fishes—that is, their dermal jaws correspond in position, but are not homologous with dermal premaxille, maxille and mandibles. The head is slightly movable on the shoulder, as in Arthrodira, and the jaws could apparently move laterally as well as vertically. Patten (1912) holds that they were bottom- 122 ANNALS NEW YORK ACADEMY OF SCIENCES Fieure 3 1. Outline restoration of Coccosteus, a primitive arthrodire. After Patten. The well-armored head and thorax are sharply differentiated from each other. They are connected by a peg-and-socket joint in the neck region. 2. Outline restoration of an antiarch, Bothriolepis. After Patten. Head and thorax are well defined, but the head is less movable than in Coccosteus- Very probably the oral and the branchial pouches with their respective muscles were becoming differentiated from each other. 83. Longitudinal section of Bothriolepis, showing the lack of endoskeletal structures, such as calcified visceral arches and the dermal character of the functional jaws. After Patten. 4. Outline of Pterichthys. After Patten. ADAMS, PHYLOGENY OF THE JAW MUSCLES 193 feeders and either took slow-moving animals that they could engulf or fed on vegetation. Patten says that they were herbivorous, for he found carbonaceous residue in their bodies. The right combination of cartilage jaws covered with bony dermal plates was first worked out by the ganoid fishes, ancestors of the modern fishes on the one hand and of the land-living vertebrates on the other. DINICHTHYS Plate XI, Figs. 1, 2 The great arthrodires of the Devonian made the best pregnathostome attempt to form a mouth with skeletal supports, but they made the fatal error of trying to form those supports solely from the bony plates of the skin rather than from the branchial cartilages. Coccosteus and Dinich- thys both show in the skull a fine apparatus that serves for seizing their prey. The arthrodiran jaw must have been a formidable weapon, as it was armed with great sharp bony projections analogous to teeth. That these great jaws were much used for biting and shearing is shown by their worn shearing surfaces. Hussakof (1906) says: “The deep scars found on the outer side of certain Dinichthys plates also bear testimony to the savage attacks of their fellows.” * The jaws seem to have worked on the principle of the joints of the insect leg, with most of the muscles attached chiefly to the plates of the body and head. The Arthrodira made another fatal mistake in that they moved the head perhaps more than the mandible. There is a joint be- tween the head and the shoulder plates (Fig. 2), with a good peg-and- socket articulation, so that while the mandibles remained more or less sta- tionary the great head with its dermal plates moved up and down against them ; thus we have the anomaly in the vertebrates of a relatively station- ary jaw and a movable head, just as though the mandible of the gnatho- stomes was attached immovably to the sternum, while the maxille and skull moved against it. It seems surprising that in the discussions as to the relationships of the Arthrodira so little importance should have been given to the peculiar motion of the head upon the thoracic shield, which is unknown among true Pisces. The movement of the mandible of Dinichthys has been very carefully studied by Dr. Hussakof (1906), to whose kindness J owe the opportunity of studying the great collection of arthodiran fossils in the American Museum of Natural History. The movement of the head upon the mandible appeared, however, to have received too little attention, and Nhis I have accordingly studied with great care. 124 ANNALS NEW YORK ACADEMY OF SCIENCES The mandibles in the arthrodires were not stationary, but they have no sign of the ordinary piscine articulation even in the best preserved fossils (Fig. 4). They appear to have been attached to the skull plates some- what as the scapula in mammals is attached to the body, namely, by liga- ments, muscles and connective tissue. From the mechanical point of view, there seems to be three possibilities for movement in these arthro- diran jaws: first, they might have had the muscles so arranged that there would have been a synchronous movement of the head and jaws, and from a study of Dinichthys this seems to be the best arrangement, as it is the one that lends itself best to the arrangement of the muscular system ; secondly, the jaws might have been arranged so as to work against the skull, but the movable joint at the back of the head seems to make this rather disadvantageous, as the head is not a stationary structure, and part of the force of the movement would be lost, as the joint in the neck would give and the head would be pushed back until it touched the dorsal shield. The usual joint or condyle in the neck region is not prominent and must have been an unealcified cartilaginous articulation, if any- thing, for the dorso-ventral movement of the head would tend to dislocate any ordinary articulation between the head and the neck. In brief, these agnathous forms developed their dermal plates just as the gnathostomes did, but while the gnathostomes put the emphasis on the cartilaginous substratum and developed from it the. principal struc- tures of the head and jaws, thus making the membrane bones subservient to the cartilage and finally drawing them in as a covering for the car- tilage, the ostracoderms and arthrodires put all the stress on the outer dermal plates and developed the movable parts from these elements, while apparently neglecting the development of the cartilaginous viseral arches. The peculiar head structure of the Arthrodira seems to imply an equally peculiar musculature as follows: (1) The joint between the dermal plates of the head and dorsal shield implies the existence of muscles to raise and lower the head. (2) As stated above, there appears to be no surface on the mandible that could articulate with a quadrate in the ordinary piscine fashion, so that from the present knowledge of arthrodiran anatomy the adductor mandibule of the Pisces could not be applied to these forms. Thus it seems probable that any system of mus- culature that would be effective in its mechanical action would be entirely unfishlike. Accordingly, in Plate XI the musculature of Dinichthys is figured according to the mechanical requirements and follows no fish type. The movements of the head would require two large muscles in order to move the head up and down on the ginglymoid joint. One pair would be in the posterior region of the skull where the marks are plain ADAMS, PHYLOGENY OF THE JAW MUSCLES 125 FIGURE 4 Dermal jaw bones of various arthrodires, lateral surfaces; from specimens in the American Museum of Natural History. The dermal lower jaws of arthrodires were probably not connected with the upper jaw by means of quadrate and articular cartilages as in true fishes, but may have been fas- tened in the thick dermis surrounding the oral cavity. 1. Stenognathus gracilis —This represents a long-jawed specialization from a Dinich- thys-like type. 2. Dinichthys intermedius.—The shearing portion of the jaw is raised above the plane of the horizontal ramus which was probably embedded in the thick dermis. 3. Diplognathus mirabilis.—A very peculiar offshoot of the coccosteid type. The sym- physeal border of the lower jaw bears tooth-like projections which apparently indicate that each jaw plate could be twisted on the long axis in a manner impossible in true fishes (Dean). 4. Dinichthys curtus. 5. Dinichthys intermedius.—To the upper end of this specimen on the outer side is attached a triangular bone which may have served for the insertion of muscles and fascia movably connecting the mandible with the inner side of the skull, somewhat as the scapula of mammals is connected with the body. 126 ANNALS NEW YORK ACADEMY OF SCIENCES in the specimens, showing that there were large muscles there. These muscles might be called the levator capitis muscles, as they raise the head. From the insertion areas this appears to have been a double muscle, one on each side of the median line. The attachment would be under the dorsal shield, where there is a strong keel that would serve as a good insertion for so important a muscle. The depressor capitis must have arisen on the heavy, inner part of the skull, and the specimens show areas that might well have served for this purpose. 'There is a large depression at the posterior end of the inner side and a strong ridge along the side of the large paired elements that might have covered the cartilage protecting the brain and might have given a base for the muscles. The insertion of this muscle is problematical. It could hardly be inserted on the plastron, for there it would be in the way of the digestive tract. The only other likely place for the insertion of the depressor capitis muscle would be on the so-called “clavicular element,” as the muscle could not have been attached to any part of the skull itself. The problem of the musculature of the mandibles is even more of a puzzle than the musculature of the skull. This mandible has been inter- preted in various ways: as a splenial by Eastman and as some other ele- ments by various writers. Whatever its history has been, it is apparently similar to nothing in the Pisces. If it is a splenial it is utterly unlike the splenial of the dipnoans, for they all show a distinct concavity on the outer side of the splenial for the reception of the Meckelian cartilage ; and examination of the specimens in the American Museum of Natural History shows no arthrodiran that has any concavity for the Meckel’s cartilage. The splenial of Ceratodus is quite concave on the outer side and is shaped in a peculiar manner at the posterior end where the articu- lation with the quadrate takes place. There is none of this in any of the Arthrodira. If this mandible represents one of the outside bones of the enathostome mandible, the same difficulty remains, for the dipnoan shows that the outer elements are concave on the inside to make a place for the Meckelian cartilage. Thus the arthrodiran mandible seems to be some- thing quite different. From the texture of the outside of the mandible, it would appear that at least the anterior half has been on the surface, while the depressed, posterior half might have been imbedded in tissue. The shape of the posterior end of the mandible makes it very evident that it is more or less free, as is seen in the free end of the teleost maxilla or operculum, where the free end is always thin and blade-hke. From analogy, we might well conclude that the same is true in the Arthrodira. In the American Museum of Natural History a new mandible of Dinichthys, which will be fully described by Dr. Hussakof, shows a pecu- ADAMS, PHYLOGENY OF THE JAW MUSCLES 127 har triangular plate on the outer face, and at the posterior end this triangular plate is roughened for the insertion of a supporting cartilage or ligament. Very probably it articulated with the inside of the cheek plate or with some other external plate and not with a quadrate. In the mandible of Mylostoma Dr. Eastman (1906) found an irregular mass on the inner side which he interpreted as the Meckelian cartilage. But if these mandibles were articulated with a quadrate they should show some indication of it at their posterior end. If the mandible, with the strong action that must have been present, as evidenced from the great marks of the shear, were pulled against the quadrate by the usual adductor muscles of the Pisces, it is reasonable to suppose that instead of being blade-like the arthrodiran mandible would show the characteristic round- ing and blunting that comes with this kind of strain, as is shown in every mandible of Pisces where there is strong pressure in the back part of the mandible. The reconstruction of the musculature of Dinichthys mandibles, as shown in Plate XI, is worked out from a mechanical point of view, as it appeared to be impossible to adapt the ordinary piscine musculature to the arthrodiran jaws. In this reconstruction it is considered that there must have been a synchronous movement of the skull and mandibles, and the musculature is figured out on this basis. From the peculiar construc- tion of the head, it is inferred that Dinichthys and its allies are the end members of a group that moved the head and had the jaws more or less fixed. The most efficient mechanical construction seemed to require that the inner face of the mandible should be connected by ligament or muscle with the “clavicular element,” which extends forward on the inside of the mandible. If the attachment at this place were by hgament and fascia and the posterior end were appressed to the inner side of the cheek plate or to some other plate in this region and worked as the scapula works on the body of a mammal, we would have the movement desired, as the rais- ing of the head would aid in raising the back part and lowering the front part of the mandible, while lowering the head would close it. The mech- anism of this part must have been very perfect, for the shear is always in one plane, with no rounding of the edges. On the inner side of the mandible at the anterior end are depressions. that show where the mandi- bles were fastened by strong ligaments. In conclusion, the evidence gained from a study of the jaw mechanism of Dinichthys seems to favor the conclusion which has been advocated by many writers, especially Dean and Hussakof, that the Arthrodira are related by common origin with the Antiarchi. 128 ANNALS NEW YORK ACADEMY OF SCIENCES ERYOPS Plate XII, Fig. 1 The massive, frog-like skull of Pryops, a Permian stegoceph, is restored with the anuran type of musculature, though some modifications have been made because of the changes that have taken place in the anuran skull. The skulls of the stegocephalians and of the Anura are strikingly similar in general pattern; that of Hryops with the dermal roofing bones covering the whole skull; that of the Anura with these bones modified, partly eliminated, and all more or less sunken beneath the skin. ‘The large palatine vacuity is similar in both. Gregory has suggested that the ancestor of the Anura was an animal much like Fryops, but that through the changes of the geologic ages the anuran skull became simplified, specialized, and the dermal temporal covering became fenestrated, partly as a result of the action of the muscles. The pterygoid region has changed slightly and has lost the flange that in Hryops projects down in a very reptilian-like manner. The mandibles are quite similar, although Hryops did not have the backward extension of the articular for the insertion of the depressor mandibule as in the frog, and the frog has lost the promi- nent suprameckelian fossa in the mandible that is so prominent in Hryops. In the Anura the muscles are all very far back on the skull, and we may assume from the Hryops skull that its muscles had a similar position, except that to make the jaw stable it should have had an anterior ptery- goid muscle on the floor of the downwardly projecting pterygoid process. The loss of the anterior pterygoid muscle in the Anura is a peculiar spe- cialization which, very probably, had not been attained by Hryops. The restoration of Hryops gives the following muscles: MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y,) Capiti-mandibularis superficialis (C. m. s.). Capiti-mandibularis medius (C. m. m.). Capiti-mandibularis profundus (C. m. p.). Pterygoideus anterior. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY J,) Capiti-mandibularis superficialis (C. m. s.).—The masseter element would occupy the posterior part of the skull in the space under the ADAMS, PHYLOGENY OF THE JAW MUSCLES 129 squamosal. It would probably have its origin on the inner side of the squamosal and quadratojugal, with a few fibers running to the quadrate and extending ventrally ; it would be inserted on the borders of the supra- meckelian fossa of the mandible. It is on the outside of the mandible in the Anura; but from the shape of the skull and the mandible there is no chance for it to be inserted there in Hryops. The muscle is split in the Anura and so the masseter of Hryops might have had two parts, the smaller one posterior, in the region of the small quadrate. Capiti-mandibularis medius (C. m. m.).—This temporal slip of the adductor mass would be under the superficial portion and over the ptery- goid muscle. It would have its origin on the inner side of the parietal, with perhaps a slight attachment on the postfrontal. It would be inserted ‘in the suprameckelian fossa. The muscle could extend quite far in the parietal region, taking hold under the skull roof. Possibly the deep part of this mass may have had a separate slip, corresponding to the capiti- mandibularis profundus or “‘pterygoideus externus” of reptiles. Pterygoideus anterior (Pt. a.).—The pterygoid muscle of Hryops was probably not homologous with the so-called “pterygoid” of the frog, but was more probably homologous with the anterior pterygoid muscle of primitive reptiles. The pterygoid bone is quite different from that of Anura, in that it has a descending flange, much like that of the Croco- dilia, that fits snugly along the inside of the mandible and serves as a guide for it. As this flange in the reptiles is functionally connected with the anterior pterygoid, it seems probable that Hryops had an anterior pterygoid muscle that extended along the pterygoid bone as far as the orbit. A pterygoid muscle is needed here for mechanical reasons, as from the teeth one would judge that the jaw was used differently from that of Anura, and there is need of a muscle here to oppose the pull of the temporal muscles, which would tend to pull the symphysis apart. Thus a pterygoid in this region would steady the jaw and pull the jaw forward against the other muscles. It would have to be inserted in the suprameckelian fossa, possibly extending to the back part. Capiti-mandibularis profundus (C. m. p.).—There is a strong possi- bility that there was a deep slip of the adductor mass, deep under the muscles, that would correspond to pterygoideus posterior of the reptiles. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule.—The articular of Hryops does not have the posterior process and the muscle must have been inserted on the under side of the articular. In some American Museum specimens the mandible 130 ANNALS NEW YORK ACADEMY OF SCIENCES seems to show a place where the depressor could have been attached. The origin of the depressor would have been on the squamosal, alongside the otic notch, since in the Anura the origin is on the posterior limb of the squamosal and on the tympanic annulus. The Anura have a second part of the depressor arising from the dorsal fascia, and Hryops also could well have had this second part of the depressor. From the inferred arrangement of the muscles in Hryops, perhaps some explanation may be given of the changes and losses that have taken place in the anuran skull. First, the capiti-mandibularis medius was no doubt attached to the parietal region and the capiti-mandibularis super- ficialis to the squamosal. To account for the open condition of the tem- poral roof in the Anura and for the loss of the tabulare, supratemporal, squamosal, postfrontal and postorbital, we may adopt the hypothesis that these muscles were pulling against one another; that the region between them gave way and made breaks and openings which did not exist in the primitive form with its unbroken temporal covering. The region be- tween the parietal and the squamosal would le between the pulls of these muscles and the first break would appear here. The disappearance of the dermosupraoccipitals, tabulars and supratemporals was probably cor- related with the opening out of the temporal region and with the exten- sion of the temporal muscles up on to the top of the skull. LABIDOSAURUS ‘Plate XII, Figs. 2, 3, 4 This primitive cotylosaur shows the covered or roofed temporal region, which it has inherited from the Stegocephaha and from such fishes as Osteolepis, Polypterus and Ama, in which the dermal bones are still in their primitive position in the skin and not sunken below it. The quad- rate is fixed, so that in that region there is no movement. The stout teeth demand strong jaw muscles and there is ample room for a large capiti-mandibularis beneath the temporal roof. The massive lower jaw with its large suprameckelian fossa also requires large muscles. The pterygoid region and the base of the brain case are essentially similar to those of Sphenodon and imply a corresponding similarity in the pterygoid muscles. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y;,) Capiti-mandibularis superficialis (C. m. s.). Capiti-mandibularis medius (C. m. m.). ADAMS, PHYLOGENY OF THE JAW MUSCLES 131 Capiti-mandibularis profundus (C. m. p.). Pterygoideus anterior (Pt. a.). MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Capiti-mandibularis superficialis (C. m. s.).—This slip of the capiti- mandibularis would cover the deeper fibers and arise from the quadrate, squamosal and jugal and be inserted with the medius in the suprameck- elian fossa. Camti-mandibularis medius (C. m. m.).—This slip would arise on the upper part of the skull and be attached to the parietal, squamosal, and perahps to some of the bones of the otic region. It would join the rest of the capiti-mandibularis in its insertion. Capiti-mandibularis profundus (C.m. p.).—This deep slip would arise on the alisphenoid and outer face of the pterygoid and be inserted with the rest of the mass on the mandible. The whole capiti-mandibularis mass would probably be inserted in the suprameckelian fossa. It would be attached to the bones surrounding the fossa and extend down into it, after the manner of other reptiles, where the whole insertion of the capiti- mandibularis is on the inside of the mandible. Pterygoideus anterior (Pt. a.)—This muscle was no doubt strongly developed as in all typical reptiles. It arose on the under side of the pterygoid and might have extended over the upper face of the pterygoid region as in Chelydra. The form of the articular shows plainly that the muscle must have been inserted on the ventral side of the articular, for there is no evidence that it extended more than slightly into the outer face of the bone. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule (D. m.).—This muscle was no doubt arranged as in other reptiles. It arose on the parietal and extended around the squamosal to be inserted on the upper face of the retroarticular process of the articular, which plainly shows the marks of the insertion. 132 ANNALS NEW YORK ACADEMY OF SCIENCES TYRANNOSAURUS Plate XIII, Fig. 1 The reconstruction of this form was attempted after some comparative study of types like Alligator, Chelydra, Aves and Euparkeria (Fig. 5). The skull in Tyrannosaurus is secondarily monimostylic, as the quadrates and the upper jaws were less movable than in the more primitive Allo- FIGURE 5 Skull of a pseudosuchian, Euparkeria capensis. After Broom ’ The borders of the large preorbital fenestra may serve for the attachment of the anterior part of the pterygoideus anterior muscle. Around the bony margin of the supratemporal fenestra arose the capiti-mandibularis. Both the supra- and the lateral temporal fenestre gave room for the expansion of the capiti-mandibularis, while the lateral fenestra of the mandible served a like function for the lower end of the same muscle. saurus. The monimostylic type is quite stable in its musculature and the reason for this is evident, as the complicated musculature needed for the movable quadrate and pterygoid is lost when the bones become fixed. Tyrannosaurus was a huge carnivorous type with massive skull and jaws; thus the musculature must have been very heavy. ADAMS, PHYLOGENY .OF THE JAW MUSCLES 133 MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY Y,) Capiti-mandibularis (with three unseparated slips). (a) Capiti-mandibularis superficialis. (>) Capiti-mandibularis medius. (c) Capiti-mandibularis profundus. Pterygoideus anterior. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule. MUSCLES OF THE ADDUCTOR OR TEMPORAL GROUP (INNERVATED BY V,) Captti-mandibularis——This muscle is considered to have three slips, designated as superficialis, medius and profundus. They are not com- petely separated in the Reptilia. The temporal fenestrae show that this muscle had retained its freedom of movement, which it had inherited from some remote aétosaur-like ancestor. There is a long fenestra under the eye, a lateral temporal and a supratemporal fenestra. The capiti- mandibularis would probably conform to the Alligator or Chelydra type. The presence of the supratemporal fenestra means that the temporal or medius slip extended through and was attached to the parietal crest. Thus the medius slip probably arose on the parietal, extended through the supratemporal fenestra and under the superficial slip to be inserted on the coronoid or in the suprameckelian fossa which is large. The superficialis slip would have fibers extending posteriorly over the rest of the mass. This slip would arise on the squamosal, quadratojugal and be inserted along the upper edge of the mandible and in the suprameckeliar fossa. In a few forms this muscle is partly inserted on the outside of the mandible, but the outside insertion is small in typical reptiles. A deep ship might have been present, representing the capiti-mandibularis pro- fundus. Pterygoideus anterior—The insertion of this muscle is plainly indi- cated in the mandible of specimen No. 5027, American Museum, which shows the place where the anterior pterygoid was wrapped around the posterior end of the jaw as in typical reptiles. The muscle probably arose in the space below and in front of the orbits and above the palatines and pterygoid, as in Alligator. The existence of antorbital fenestre was held by Dollo and by Gregory and Adams (1915) to be correlated with the an- terior extension of the anterior pterygoid muscle—a view adopted here. 134 ANNALS NEW YORK ACADEMY OF SOIEBNOES MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP UNNERVATHD BY VII) This muscle is constant in the Reptilia and its insertion is evident on the specimen No. 5027, American Museum. In many reptiles where the back part of the skull is not greatly modified its origin is on the parietal, but here it seems to have been on the paroccipital. The muscle arose on the ventral end of the paroccipitals, the ventral ends of which are rough- ened for a muscle insertion. Its insertion on the mandible was on the articular, posterior to the articulating surface, where there is a depression similar to that seen in most reptiles. CYNOGNATHUS Plate XIII, Figs. 2-5 The skull is reptilian as a whole, but parts of it very plainly show the mammalian characters, especially the squamosal, the jugal, the dentition and the basicranial region. The quadrate is reduced. The jaw, although reptilian, is of a type that foreshadows that of mammals. 'The dentary had enlarged from the primitively slender dentary of the early reptiles until it was the most important part of the mandible and must have car- ried the greater part of the musculature. The coronoid process of the dentary is large and ascends far into the temporal fossa. The angle of the dentary is developing and shows plainly. The posterior part of the jaw, consisting of the articular and other elements, is becoming small and is so loosely attached to the dentary that in fossil specimens it is usually missing or found separate from the skull. The old reptilian articulation with the skull had become reduced, but was still functional. Although the new articulation with the jaw had not yet been formed, I infer, from the shape of the temporal fossa and from the direction of the muscles, that the jaw was pulled not directly against the quadrate but toward a point above it on the squamosal. MUSCLES OF THE ADDUOTOR OR TEMPORAL GROUP (INNERVATED BY YV,) Capiti-mandibularis. (a) Capiti-mandibularis superficialis. (b) Capiti-mandibularis medius. (c) Capiti-mandibularis profundus. Pterygoideus anterior. ADAMS, PHYLOGENY OF THE JAW MUSCLES 135 MUSCLES OF THE DEPRESSOR OF DIGASTEIC GROUP UNMHERVATED BY VII) MUSCLES OF THE ADDUCTOE OF TEMPORAL GROUP USSERVATED BY V,) Capitt-mandibulariz—In Cynognathus this mass of muscle, which is the reptilian muscle of the same name, must have been well subdivided; _ thus there was probably a large temporalis or medius slip filling the large temporal fossa and a superficial or masseter slip arising along the zygo- matic arch, both outside and inside, both muscles being attached on the large dentary. The large carnivorous jaws of this form indicate a corre- spondingly powerful muscle which not only filled the temporal fossa but also must have been attached to the outside of the zygomatic arch. The attachment must have been on the ascending process of the dentary, for the posterior bones of the jaw are too small. Watson (1912, p. 531) says: It is a remarkable fact that in Cynodonts increasing size of the dentary and of the masticatery muscles. which in Cynognathuz roust be inserted om it. is correlated with a reduction and weakening of the back part of the jaw. which alone articulates with the skull: not only are the actual bones small but their attachment to the dentary is weak: they merely rest in the groove in that bone and are often displaced in the fossil skulls As the ascending process of the dentary affords ample space, the capiti- mandibularis must have been attached chiefiy to this bone, as in mammals. (a) Capiti-mandthularis superficializ—On the outside of the dentary there is a large fossa for the masseter, which must also have been large to match the large temporal mass. It probably had a hold on the inner and outer side of the zygomatic arch, extending forward to the large anterior tubercle on the lower border of the zygoma. The attachment is probably in the depression on the outside of the dentary. From the condition in other reptiles, this superficial slip could extend partly over the rest of the mass and be connected with the fascia of that region. As mentioned before, the pull of these muscles would tend to pull the jaw upwards against the squamosal and not so much against the quadrate. This gave the posterior part of the jaw the opportunity to complete its separation from the dentary im some more advanced form of theriodont and to enter on its new mammalian function in connection with the middle ear. (6) Capiti-mandibularis medius—This part of the temporal mass would fill the temporal fossa and be inserted on the inner and outer sides of the dentary. 136 ANNALS NEW YORK ACADEMY OF SCIENCES (c) Capiti-mandibularis profundus.—Vhis deep slip must have taken its origin on the inner side of the temporal mass and have been inserted on the ascending process of the dentary. Pterygoideus antertor—To counterbalance the pull of the capiti-man- dibularis mass the anterior pterygoids must have played an important part. The ascending process of the pterygoid is quite large, thus giving an at- tachment for large pterygoids. There is no need for the complex mus- culature of the type found in streptostylic reptiles, so it was probably more of the chelonian or rhynchocephalan type. The pterygoid bones of Cynognathus are bowed inward to the mid line, covering the primary skull base and forming on each side an extended longitudinal fossa for the origin of the anterior pterygoid muscle. Although the articular, pre- articular and angular bones are small, they seem sufficiently large for the insertion of the anterior pterygoid in the normal place at the back part of the jaw. The insertion of this muscle in all recent reptiles is at or near the rear end of the jaw, and in view of the general fact that inser- tion areas are less variable than origin areas, there seems no reason for regarding Cynognathus as having an exceptional insertion of the anterior pterygoid. The pull of the anterior pterygoid muscle would counter- balance that of the capiti-mandibularis mass and take some of the strain off the weak quadrate articulation. The dentary, imbedded in the tem- poral mass, would thus act as a cushion and the action of the jaw would be the same as though a strong articulation were present. That this action of the jaw was effective is shown by the robust character of the teeth and jaw. If Cynognathus had relied only upon the old articulation by way of the quadrate and articular, a bite on a piece of bone might have dislocated the jaw. As the dentition is carnivorous, the jaws did not have much lateral motion. The reason Cynognathus had a large ascend- ing ramus of the dentary is that it was derived from therocephalians and ultimately from primitive Theromorpha, in which the coronoid bone was functionally replaced by an obliquely ascending process of the dentary. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VII) Depressor mandibule.—This muscle was in its usual position, as the posterior end of the mandible shows the insertion. In all reptiles there is a remnant of the C,md of Ruge, and from this the mammalian digas- tric was probably derived. With the loss of the posterior end of the rep- tilian jaw the depressor mandibule of these forms would disappear and a new digastric would be formed from this sheet of muscle that has per- sisted from the fish stage. The long ventral muscles of the throat could ADAMS, PHYLOGENY OF THE JAW MUSCLES 13% give much aid in lowering the mandible. Watson (1912, pp. 581-582) Says: As it is impossible that a muscle should increase while its point of attach- ment is degenerating, it appears probable that the pterygoid muscles were mainly inserted onto the postero-inferior angle of the dentary, which is thick- ened. Thus inserted, these muscles, while tending to close the mouth, would produce stresses in the hinder part of the jaw, in the opposite direction to those induced by the masseter and temporal muscles, in this way permitting the reduction of the hinder part of the jaw which we actually see. The fact that in higher Cynodonts all the masticatory muscles have their attachments on the dentary renders the freeing of the articular and quadrate demanded by the quadrate-incus theory of the mammalian ossicula auditus much more understandable. It would seem that Watson’s hypothesis that all of the muscles were on the dentary might be questioned, for the cast of the skull of Cynognathus shows plainly the insertion of the depressor mandibule on the posterior end of the articular, while the insertion of the anterior pterygoid seems to have been in the usual position, wrapping around the end of the man- dible as in other reptiles. This last muscle may have been reduced, but its insertion areas on the articular and prearticular seem evident. HOMOLOGIES OF THE JAW MUSCLES IN VERTEBRATES (Tables I-V) INTRODUCTION The first attempts to homologize the jaw muscles in the different groups of vertebrates were based purely upon similarities of function and posi- tion; and while the work was useful and gave a start in the right direc- tion, it was not altogether rehable. The older writers quickly took up the innervation, as its importance in comparative anatomy became known, and applied it to the determination of the muscles. Thus a much more certain classification arose. Most of the work has been on restricted groups. Vetter in his works on the elasmobranchs and other fishes gave for the muscles of these groups the names that are still used. He divided the muscles up into groups according to their immervation and position and gave the best basis for the classification of the piscine musculature. One of the few papers dealing with the entire vertebrate group is a dissertation by Dr. Ernst Teutleben, published in 1874 under the title, Ueber Kaumuskeln und Kaumechanismus bei den Wirbelthiere. He ex- amined a series of vertebrates and gave a very good description, for the 138 ANNALS NEW YORK ACADEMY OF SCIENCES time, of the jaw muscles in some of the common vertebrates; he also made some observations on the mechanics of the jaw action. He studied the following forms: Dog, horse, sheep, porpoise, bird, Crotalus durissus, alligator, frog, pike and whiting. He failed to differentiate many of the muscles and made no mention of innervation, which is one of the chief criteria used at the present time. The names applied were based upon the origin and insertion of the muscles. He applied this principle to the different classes and gave the muscles names. He did not attempt homol- ogies, except as based upon similar function. He misinterpreted the detrahens mandibule of Ornithorhynchus, as all the others had done until the neurology was worked out by later workers. ~ The greatest stimulation to the work was given by the researches of Ruge on the facialis nerve. He studied this throughout the vertebrates from the elasmobranchs to mammals and gave a reliable basis for the’ determination of the muscles of this group. Schulman, in his work on the trigeminus musculature of the monotremes, cleared up some of the puzzles that this aberrant group present. Lubosch, Fiirbringer and others have added much to this work, so that the comparative anatomist now has extensive material for comparison. Gaupp in his work on Reichert’s theory of the origin of the auditory ossicles and Versluys in his studies on the auditory organs in reptiles have worked in this rich field and have given the material a definite meaning, especially in some of the trouble- some problems relating to the changes that took place in the shifting of the bones and muscles, when reptiles of some sort were changed into mammals. Many other investigations have given much information on the muscu- lature of special forms of vertebrates. Chaine, Rouviére, Bijvoet, Toldt, Parsons and others have collected the necessary data on the digastric muscle and have given very full accounts of the condition of this muscle in the mammals. Toldt, in his paper on the jaw articulation and its problems, gives us a basis for the classification of the types of vertebrate jaws, of their articulations and of the correlated types of musculature. Apparently the present work is the first to give a general illustrated re- view of the jaw muscles of vertebrates and to apply this knowledge to an interpretation of the skull structure of recent and fossil types. The study of 26 different forms, representing the classes Pisces, Am- phibia, Reptilia, Aves and Mammalia, has demonstrated that the muscle masses in general are severally homologous from the Pisces to Mammalia, and that they may be grouped into two great systems: First, the muscles innervated by the ramus mandibularis trigemini V,, and, second, the muscles innervated by the facial nerve (VII). There is a sharp line ADAMS, PHYLOGENY OF THE JAW MUSCLES 139 between these muscular systems and their general position has been the same throughout. The trigeminus innervates the muscles of mastication that le in front of the quadrate, while the facialis innervates the muscles behind the quadrate and those of the hyoid region. These regions in the mammals overlap on the surface, for the reptilian sphincter colli has crept forward over the deeper muscles of the trigeminus and has formed the mimetic muscles of the face, so that while the deep muscles, anterior to the hyoid arch, are innervated by the trigeminus the superficial muscles are innervated by the facialis. This is an example of the faithfulness with which the nerves follow the muscles in their migration (Ruge, 1897). Starting with the elasmobranchs, we find the adductor mandibule muscles, which are undifferentiated masses, derived from the pro-branchial muscles before the anterior branchial arches were transformed into jaws. The adductors are not separated into special muscles, but represent the “mother mass” of the chief jaw-muscles of the V, group in all the higher classes of vertebrates. A depressor mandibulx is not differentiated in the elasmobranchs or in the other classes of Pisces, but is. represented by the second dorsal superficial constrictor throughout this group, a true depressor first appearing in the Amphibia. In the development of the Amphibia from some of the Pisces the jaw musculature was carried over in its general plan, but the muscle masses took on the peculiar modifications needed in each group. We may con- ceive that these “mother masses” C, and C, became differentiated in many ways throughout the different classes of vertebrates, and that various slips were given off from these masses which served their purpose and the needs of the animal, only to be dropped in the future development, while new slips arose in the higher forms. We may further conceive that these mother masses were innervated by the nerves V, and VII, and that in the first stages of the origin of a new muscle slip there was a mere branch- ing of the nerve into small twigs; in this stage there was no differentiation into special nerves and no division of the muscles into separate slips, but merely partially separated portions with the nerve twigs following and gradually becoming more differentiated, as conditions demanded further separation. Finally, when these shps were separated off from the main mass, the nerve twig became a branch, and the muscle, having lost all connection with the fibers of the parent mass, might be called a separate muscle. This is my conception of the origin of the special muscles in the different classes, and with this view the precise homology, except within the class, is sometimes doubtful, unless very primitive or annectent forms between widely separate groups are available for study. 140 ANNALS NEW YORK ACADEMY OF SCIENCES This separation of the muscle slips from the mother mass may be shown in embryology; for the muscle groups start as undifferentiated masses and then become divided into the different slips. Lubosch (1913) de- scribes the separation from the “temporal mass” of a slip which becomes the “pterygoid” of the urodele. This splitting of the muscle is quite definitely known, and in this paper I have made frequent use of this fact in endeavoring to determine the homologies of the different groups and in reconstructing the muscular systems in extinct forms. (See chapter on reconstructions. ) Some individual muscles may, however, be followed through all the classes of vertebrates. If the history of a bone has been traced from the Pisces to the Mammalia, there is no reason for assuming that the muscles associated with it have changed, provided that they are present in all of the classes and have retained their origin, insertion and to a certain ex- tent their function. Perhaps the hyomandibular (= stapes), the pre- opercular (—squamosal), and other bones whose history is pretty well known, might be considered as having taken their muscles with them throughout the evolutionary changes from the fishes to mammals, if there is no mechanical or other reason for the dropping of the old and develop- ment of new muscle shps. Muscles are pliable tissues having the power of changing, either by shifting their origin or, if there is no demand for their service, by dropping out. Vestigial muscles found in each of the classes have been carried over from an earlier class and have lost their usefulness and atrophied. A long list of such muscles might be com- piled from the Mammalia or from any class. The vestigial muscles of the ears in Homo, carried over from the simian stage, vestigial muscles in birds, carried over from the Reptilia, the vestiges of the levator arcus palatini in Cryptobranchus and Amphiuma, carried over from the Pisces (Lubosch, 1913, p. 71), the “adductor maxille” in Cryptobranchus from the Pisces, are all vestigial and more or less functionless muscles carried over from one class to another. HoMoLOGY OF THE JAW MUSCLES IN THE PISCES (Table I) The primary division of the musculature may be made with the inner- vation as a guide, as there is a natural grouping of the muscles of the head into two systems—the muscles innervated by the fifth or trigeminus and those innervated by the seventh or facialis nerve. The muscles also divide into the same two divisions if we group them first as muscles an- terior to the quadrate region and secondly as those posterior to it. The . ADAMS, PHYLOGENY OF THE JAW MUSCLES 14:4 exceptions to this grouping are the mimetic muscles of the face in mam- mals, which represent the muscles from the second constrictor, innervated by the facialis, that have extended over the face and taken their nerves with them. If we assume that the jaw and hyoid elements represent the first two visceral arches that have changed their function in some pregnathostome stage, it is also fair to assume that the muscles went with them, and that the jaw muscles represent the much divided and specialized constrictors and adductors of primitive branchial arches. Vetter’s analysis of the muscles of the visceral arches appears to hold good at the present day. He held that the adductor mass of the jaws of fishes was homologous with the “adductores arcuum visceralium,” which were small muscles on the inner side of the branchial arches, “mittlere Beuger der Bogen,” lying between the dorsal constrictors above and the ventral constrictors below. As these branchial arches changed into jaws and the development of the hyomandibular, quadrate and opercular elements proceeded, the con- strictor and adductor muscles, which were already in position, would re- quire but little change to take on new functions as jaw and opercular muscles. Accordingly, the constrictors of the pre-gnathostomes may be regarded as the primitive head muscles. In the elasmobranchs the con- strictor became divided into different sections, so that there were dorsal, median and ventral sections, which were either further differentiated to be used as needed or held in reserve for future changes, as in the case of the digastric muscle. This is represented in the elasmobranchs (Ruge, 1897) by the undifferentiated second constrictor, which retains its con- strictor-like form until it becomes specialized in the amphibians into a definite muscle. Throughout the gnathostomes we find muscle masses that may be ho- mologized, but the homology of the separate slips is often questionable. In this paper the attempt is made to work out the homology of the main muscle masses (“Mother” masses) first and then, if possible, to point out the homologies of the separate slips of each mass, especially in the mem- bers of the same class and where possible between classes. To homologize the separate slips through the different classes means that cne must know the ancestry and be able to demonstrate the movements and changes in the osteology, so that in default of such knowledge the result is often ques- tionable. The muscles of the piscine head may be divided as follows (the related muscles are placed together) : 142 ANNALS NEW YORK ACADEMY OF SCIENCES MUSCLES INNERVATED BY THE TRIGEMINUS NERVE Levator maxille superioris. Levator arcus palatini. Protractor hyomandibularis. Dilator operculi. Adductor mandibule (often divided in the Pisces). MUSCLES INNERVATED BY THE FACIALIS NERVE Adductor hyomandibularis. Levator opercull. Adductor operculi. MUSCLES INNERVATED BY THE TRIGEMINUS NERVB Levator maxille superioris (Adductor B Vetter). Levator arcus palatint 1-5 McMurrich.—These are a series of muscles that extend from the under side of the skull to the maxilla. They vary in number in the Pisces from one to four or five. In Acanthias they arise in a mass anterior to the spiracle and the remains of the first constrictor dorsalis superficialis and extend to the maxillz; here they form a single muscle. In Ama they are partly anterior to the eyes. They probably represent the dorsal part of the first constrictor superficialis and function in the movement of the maxille. In the teleosts they lose their impor- tance and disappear, except for a few tendinous remains, while the levator arcus palatini becomes more important and usurps the place of the ley- ators of the maxille. Levator arcus palatint.—This muscle raises the bones of the palatine region and is of great importance in the fishes where this region is de- veloped. In the elasmobranchs it is not needed, but is perhaps repre- sented by the remains of the first constrictor, just anterior to the spiracle. It is prominent in most teleosts and Holostei, but drops out where this region becomes fixed. This muscle is closely related to the protractor hyomandibularis and the dilator opereuh. McMurrich (1885) calls the protractor hyomandibularis “levator arcus palatini” in Amia. They are often close together and sometimes not entirely separated as in Amita. Protractor hyomandibularis.—This draws the hyomandibular forward. It probably represents a part of the first constrictor. It is always closely connected with the levator arcus palatini. It varies in size, being very large in Acipenser and Polyodon, of good size in the teleosts, and absent in the dipnoans, where the hyomandibular is rudimentary or absent. Dilator opercult.—This is the posterior slip of the first constrictor. It pulls the operculum forward and slightly raises it. It is closely con- ADAMS, PHYLOGENY OF THE JAW MUSCLES 143 nected with the protractor operculi, always lying just behind it, extend- ing over the groove for it in the hyomandibular just above the preoper- cular. Often the preopercular also shows where the muscle extends over the upper end of it. Practically every skull studied showed this muscle area, so that it is easily demonstrated on any skull in the teleosts or other fish group. It is inserted on the under side of the opercular in most forms. Adductor mandibule.—This is considered as the mother mass of the chief jaw muscles throughout the vertebrates. In the elasmobranchs it is almost a single mass, partially subdivided in the rest of the fishes and reptiles and completely divided in the Amphibia and Mammalia. It is innervated by the third branch of the trigeminus and represents a part of the first constrictor with its nerve. The condition in the elasmo- branchs is usually as a single mass, although the fibers cross each other and do not always extend in the same direction, but there is no definite separation in those studied and none described in the literature on other forms. There is a tendency throughout the remaining Pisces for this mass to be partially divided. In general this muscle may be divided into two parts—a superficial part that extends across the other fibers and is attached in the quadrate and squamosal region and a deeper set of fibers which extend up to the postfrontal and parietal region. No attempt is made in this paper to homologize these portions, although they are con- stant and seen to be starting a condition that eventually may have re- sulted in the separation of these slips from the mother mass. The sim- plest adductor mandibule is found in Acanthias and the most complex in Amia and EHsoz, although the division is never complete. Amia, Pali- nurichthys and many other forms show a peculiar specialization, where a part of the adductor (Adm*) extends into the suprameckelian fossa and excavates the dentary, so that the bone is completely filled with this part of the muscle. MUSCLES INNERVATED BY THE FAOIALIS NERVE This series of three muscles is concerned with the movement of the opercular and hyomandibular bones. They represent a part of the second dorsal constrictor and show the characteristic innervation of this mass. Adductor hyomandibularis.—This is a deep muscle which arises on the posterior part of the skull in the otic region, anterior to the other two. It is usually small and short and is not easily seen unless the opercular bones are removed. It opposes the protractor hyomandibularis and raises the hyomandibular bone. It is attached to the inner side or to the pos- terior border. It is shown in Acipenser, Polyodon and Polypterus. 144 ANNALS NEW YORK ACADEMY OF SCIENCES Adductor opercult and levator operculi.mThese are closely associated. Both arise on the posterior part of the skull in the otic region and are inserted on the inner side of the opercular bone. Often one of them is absent or perhaps they are not differentiated. They are present in almost all the Pisces with the exception of elasmobranchs. They are small in the dipnoans. HomoLoGy oF THE JAw MuscLES IN THE AMPHIBIA (Table IT) There is quite a difference between the muscles of the Pisces and those of the Amphibia, for in the latter muscles masses have become more spe- ciahized by a splitting off of the different slips, so that they may be called separate muscles. Some of these divisions were suggested in the Pisces by the direction of the fibers and by differences in the origin and inser- tion, but they remained a part of the parent mass, as they do, for the most part, also in the Reptiha. In the Amphibia the muscles of the anterior part of the piscine head have disappeared, being represented by vestiges only. Lubosch (1913, p. 71) says: s$isher unbekannte Muskelrudimente wurden gefunden bei Amphiuma und Cryptobranchus. (1) Ein M. levator arcus palatini bei Amphiuma und Cryptobranchus, von der knorpligen Nasseskapsel und (Amphiwma) der vertikalen Lamelle des Frontale (Wiedersheim) entsprigend und zur Membrana pterygomaxillaris zeihend. (2) Ein M. adductor maxille bei Cryptobranchus vom vorderen Rand des kné6chernen Pterygoids und dem knorpligen Proc. pterygoideus quadrati ent- springend und in der Nihe des Maxillare in der Membrana pterygo-mavxillaris endend. Beide Muskeln werden mit feinen Aestchen aus demselben Nerven versehen, whelcher auch die Mm. pterygoidei versoret. The great changes in the skull of the Amphibia account for the reduc- tion and dropping out of several of these typical piscine muscles. There is no need for the levator maxillze superioris and the levator arcus pala- tini, as the parts controlled by these muscles are fixed. The new form of the bones demand a different musculature and the loss of others. The preopercular, symplectic, hyomandibular, opercular, inter- and sub-oper- cular and several of the bones of the skull and maxillary region have either been lost or changed their functions, so that new muscles are needed. If the hyomandibular is considered to be the stapes of the higher forms, it seems to have discarded its original musculature in the transfor- mation, for there are no muscles in the forms with a stapes that could have been retained from the muscles of the hyomandibular. The two the protractor (V.) and the levator (VIIT)— hyomandibular muscles ADAMS, PHYLOGENY OF THE JAW MUSCLES 145 could not have been carried through to the Mammalia, or at least there is nothing to show that such was the case, as the new stapedial muscle is regarded as a slip from the C,md or from the reptilian depressor man- dibula. The preoperculum, which is considered to be the squamosal of the forms above the Pisces, retains its old connection with the temporal muscle mass, so that the masseter (or superficial) slip of the urodeles and anurans is still associated with the transformed preoperculum. In the Amphibia the piscine operculum is absent, so that the three muscles attached to it in the Pisces would be lost. The simplest condition of the musculature is found in the urodeles, where the muscles are well separated, but are not so specialized as in the Anura and retain more of their piscine condition. They are divided into slips, three or four in number. These are separate slips that compare with those indicated in the muscles of the reptiles, but are not regarded as fully homologous with them. The superficial slip is the typical mas- seter-like muscle with the same general direction of the fibers and the same relation to the main mass. ‘The deeper muscles of the peculiar “Temporal mass” have a peculiar specialization, in that one slip extends back over the skull and arises no longer on the parietal, but on the neck vertebra, sometimes as far back as the fourth cervical vertebra. (See urodeles.) ‘The anterior temporal, which arises from the side of the skull, behind the eyes, and runs outward and backward to the inner side of the mandible, is apparently comparable to the pterygoideus anterior of rep- tiles. This pterygoideus anterior has separated from the under side of the capiti-mandibularis mass, but does not arise from the pterygoid. The Anura show the highest specialization found in the amphibians, as the muscles are not only divided into slips, but the slips are well sepa- rated, both in their origins and in their insertions on the mandible. 'The striking thing in connection with the anuran jaw muscles is their con- nection with the auditory region, for the muscles of mastication and some of the neck muscles serve to keep the tympanum stretched taut. The division of the capiti-mandibularis superficialis (masseter) is suggested in the urodeles, where the fibers of the single muscle show an interme- diate condition. The depressor mandibule is the same throughout the group. It has two slips, one from the skull and one from the dorsal fascia. They represent again the C,md of Ruge, which is the mother mass of this group innervated by the VII nerve. It seems from the innervation, origin and insertion that the individual slips may be homolo- gized throughout the Amphibia, and the homologies as they appear are shown in Table II. 146 ANNALS NEW YORK ACADEMY OF SCIENCES HoMOLOGIES OF THE JAW MUSCLES IN THE REPTILIA : (Table IIT) The jaw muscles of the Reptilia and Aves show the same division into masses as do those of the Pisces and Amphibia. There is the same group- ing of the masses into muscles innervated by V, and by VII and they have the same relative position as in the lower forms. The muscles show more specialization, as there is a higher specialization of the skull in Reptilia and Aves and consequently a more complex musculature. In the reptiles the capiti-mandibularis mass is more homogeneous than in the Amphibia, and while the pterygoid muscles have separated off, the capiti-mandibularis superficialis remains attached. In the birds there is a complete separation of the muscles. The great specialization in the higher or streptostylic reptilian skulls is correlated with the great complexity of the musculature. The Reptilia are divided arbitrarily into two groups according to the fixity or mova- bility of the quadrate; but this is a relative division, for all stages are found between the streptostylic and the monimostylic conditions. The jaw muscles of the Reptilia may be grouped according to the innervation, as follows: MUSCLES OF THB OCAPITI-MANDIBULARIS OR TEMPORAL GROUP (INNERVATED BY Y;) Capiti-mandibularis. (a) Superficial slip (not separate). Masseter. (b) Medius slp (not separate). Temporal. (c) Profundus slip (not separate). Deep layer. (d) Cranio-pterygoid. Pterygoideus anterior. Pterygoideus posterior. MUSCLES OF THE DEPRESSOR OR DIGASTRIC GROUP (INNERVATED BY VIT) Depressor mandibule. MUSCLES OF THE CAPITI-MANDIBULARIS GROUP (INNERVATED BY YV,) Capiti-mandibularis.—This muscle is not divided into separate muscles, but the conditions suggest the divisions to appear later in the mammal- like reptiles. (a) The outer or superficial slp may apparently be traced from the fish to the mammal. It always arises from the squamosal region ADAMS, PHYLOGENY OF THE JAW MUSCLES 147 and is inserted on the coronoid region of the jaw along with the rest of the mass. (b) The medius slip also is not separate, the fibers being under the superficial slip and extending to the parietal region. (c) The profundus is represented by the deep part of the mass. (d) The cranio- pterygoid muscles are the special muscles developed in the pterygoid re- gion of the streptostylic reptiles and are new developments. Pterygoideus anterior and pterygoideus posterior.—The so-called ptery- goid muscles of reptiles include two quite different groups; the posterior group represents the deepest part of the capiti-mandibularis mass. It is usually named the pterygoideus externus or external pterygoid, but it is probably only partly homologous with the mammalian external pterygoid. I have named it the capiti-mandibularis profundus. It is often sub- divided into two heads, which may be named caput 1 and caput.2. The anterior pterygoid called by Mivart “Internal pterygoid” and in this paper “Pterygoideus anterior” represents the anterior part of the ad- ductor mass or C,smd of fishes and is foreshadowed in Polypterus. It may be the homolog also of the so-called “temporalis” of urodeles. It is far in front of the “Pterygoideus externus” and runs in the opposite direction, crossing it at a wide angle on its way from the orbital region to the back of the mandible. It is always present in the reptiles and acts in concert with the capiti-mandibularis mass in closing the jaw. More in detail the action is described in the section on reptiles. MUSCLES OF THE DEPRESSOR GROUP (INNERVATED BY VII) The depressor mandibulz acts as the depressor of the mandible. It is constant throughout the Sauropsida. It was retained throughout the series until the mammal was evolved, when probably a new slip was sepa- rated from the mother mass and the new depressor of the mammal was formed. Futamura (1907, p. 570) on comparative and embryological evidence stated that the stapedial muscle of the mammals and man comes from the proximal or upper part of the muscles of the hyoid arch (innervated by VII)—that is, the mother mass of the depressor mandibula—and that the reptiles also derive their stapedial muscles from the same source. Futamura says: Der M. stapedius stammt von den Muskelfasern die sich yom proximalen Teil der Hyoidmuskulatur nach Gehérkndchelchen begeben: Bei den Reptilien (Krokodil, Lacerta) ist er mehrfach und in seine Funktion auch verschieden von der der Végel und Siiugethiere. Nach Killian finden sich beim Krokodile Ohrmuskeln, die Heber, Herabzieher der Ohrklappe und Spanner des Trommel- 148 ANNALS NEW YORK ACADEMY OF SCIENCES felles sind. Bei der Hidechse konnte ich nur zwei Muskel finden, die sich dem Levator der Ohrklappe und dem Depressor der Ohrklappe und Spanner des Trommelfelles der Krokodile verglichen habe. Bei den V6geln, Siiugethiere und dem Menschen ist der Stapedius einfach, geht beim Vogel an der Columella und bei den Siiugethiere an den Steigbiigel. Hier verliert er damit die direkte Beziehung zum Trommelfell und macht auch einen Funktionswechsel durch, auf den ich hier aber nicht niiher eingehen will. SUMMARY The relations of the jaw muscles of reptiles and Amphibia are well summarized by Lubosch (1913, pp. 72-73) as follows: Die Vergleichung mit den Reptilien (verg. die Beschreibungen yon Fischer, d’ Alton, Sanders, Mivart, Versluys, Watkinson, Bradley) Der Organization- stypus der Urodelen ist auch heir unverkennbar vorhanden. Die Muskulatur ist zwar missiger, aber zweifellos weiniger eingreifend gegliedert als dort. (Bei Krokodilen ist sogar Masseter und Temporalis sehr reduziert.) Der Ramus mandibularis liegt auch hier zwischen Masseter und Temporalis aussen, Pterygoideus innen. Der Pterygoideuskomplex tritt auf als Pterygo-mandib- ularis (Bradley) (homolog dem Pterygoideus posterior der Urodelen) und als ein als, Pterygoideus (Bradley) auch “Pterygoideus internus” bezeichneter Muskel (Homolog dem Pterygoideus anterior der Urodelen) Bei Cheloniern (und Krokodilien . . .) sind alle Hauptteile der Muskulature scharf geson- dert, bei Lacertiliern und Ophidiern scheinen sie nach den Angaben der Liter- ature mehr zusammen-zuhiingen. Besondere EHEigentiimlichkeiten bietet eine tiefe Masseter-portion dar, in betreff welcher auf die genauere Darstellung verweisen wird. Wichtig is nun vor allem die Differenzierung der motorischen Trigeminusiiste. HoMmMoLoGy OF THE JAW MUSCLES IN AVES (Table IIT) The musculature of the jaws in birds has been derived from the rep- tilian position with little change. As the bird skull is streptostylic, the reptile most easily compared with it is Varanus and some of the fossil forms as mentioned in the section on Aves. There is a separation of the muscles, so that the capiti-mandibularis is divided into a superficial, a median and deep portion. The deep portion fills its usual place and the superficial has its origin on the squamosal and quadrate. These muscles are opposed by two pterygoid muscles that are homologous with those of the reptiles. The pterygoideus anterior (pterygoid internus) is the homolog of the muscle of the same name in the Reptila; it does not wrap around the end of the mandible but is inserted on the inner, posterior face of the mandible as it does in some reptiles—e. g., Chelonia. Pterygoideus pos- ADAMS, PHYLOGENY OF THE JAW MUSCLES 149 terior is the posterior slip from the capiti-mandibularis, which is sepa- rate in birds. It crosses the anterior pterygoid and is the homolog of the muscle of the same name in Reptilia. The depressor mandibulx is large and is the homolog of the depressor of the Reptilia. The presence of the stapedial muscle in the Aves shows that in some of the reptiles and Aves the stapes was connected with a part of the de- pressor mandibule. Futamura (1907) has shown that this was true in the embryology of the sparrow and of the duck: Der Stapedius entspringt von der Gehérkapsel und geht zum Columella und zu der medialen Seite der Gehidrgrube. Ein Teil der Muskelfasern scheint mit dem proximalen Abschnitt des Quadratum Beziehung zu haben. In der Sperling der M. stapedius, der aus dem yvorher erwiihnten proximalen Teil der Facialismuskulatur entsteht, ist sogar schon ziemlich deutlich differ- enziert. Er liegt an der dorsalen Seite der tibrigen Facialismuskulatur und zieht grésstenteils an der medialen Seite des Facialisstammes nach hinten, medial, und endet dicht an der Carotiswand der Nervus facialis ist bedeutend verlingert. In der Ente M. stapedius, digastricus und stylohyoideus differen- zieren sich aus dem Blasten der post-auricularen Gegend. HoMOLOGIES OF THE JAW Muscies IN MAMMALS (Table IV) The great transformation of the reptilian skull into the mammalian skull has been correlated with equal changes in the jaw muscles. The temporal fosse of mammals haye opened out, so that the temporal and masseter muscles extend to the surface of the head and thus gain room and free action for diverse adaptation and for expansion. By the devel- opment of the sagittal and occipital crests the areas for the origin of the temporal and neck muscles are greatly increased. The masseter becomes enlarged and its origin spreads over the zygomatic arch. This specializa- tion of the superficial part of the temporal mass is typically mammalian, as nothing like it is seen in the reptiles. The great changes in the mandible, the reduction of the posterior jaw elements, and the development from them of the malleus and incus have been correlated with an equal transformation of the muscles. The inser- tion of the whole temporal mass has shifted from the coronoid region of the mandible to the dentary, while the external muscle gave rise to both the internal and external muscles of the mammals. The formation of accessory auditory ossicles from the reduced posterior jaw elements gave opportunity to certain of the reptilian muscles to assume new functions. The stapedial muscle, for example, would be homologized with the 150 ANNALS NEW YORK ACADEMY OF SCIENCES upper part of the reptilian depressor mandibule, as it has been shown in the sections on birds and reptiles that the depressor is in the position to gain the relations of the stapedial muscle of the mammals. It hardly seems possible, from the conditions seen in Cynognathus, that the de- pressor mandibule has been carried over from the reptiles as the posterior belly of the mammalian digastric. The most satisfactory conclusion ap- pears to be that a part of the depressor became the stapedial muscle and that the mammalian digastric is a new slip from the second constrictor mass, which was still retained in the reptiles (Ruge). Wilder (1909), in discussing the origin of the stapedius, says: “A portion of the posterior belly, that is, of the second levator, becomes separated from it in the rep- tiles, and follows the stapes into the middle ear, whence it becomes the stapedius muscle, innervated by a special branch of the facialis.” That the stapedius muscle is an ancient muscle appears to be shown by the great separation of the digastric and stapedial nerves, both of which are branches of nerve VII. In other groups of related muscles the nerves, although branching and becoming more than mere nerve twigs, still indi- cate their relationship by their proximity to each other, as in the case of the subdivisions of the nerve V,, where the muscles that are closely related have their nerves coming off close together. The stapedial nerve comes off at the upper part of the facialis, while the digastric nerve comes off far below it. This seems to help the hypothesis of the derivation of the stapedial and the wide separation in time from the appearance of the mammalian digastric. Two muscles, the tensor tympani and tensor palati, appear in the mam- mals for the first time. They are innervated by the ramus mandibularis of the trigeminus and, according to Gaupp, their homology may be looked for in one of the pterygoid muscles of the reptiles, namely, the “pterygo- mandibularis” of Bradley, which is the “anterior pterygoid” of the pres- ent work. This muscle is in the position that would permit it to be drawn into the middle ear as the tensor tympani, and it is also in a posi- tion to give rise to the tensor palati. The anterior pterygoid of reptiles is attached to the posterior end of the mandible, and during the change from the reptilian to the mamma- lian condition it could be drawn into the middle ear without much change, as has been shown above. A review of Gaupp’s discussion, however, leaves me in doubt whether these muscles have both come from the reptilian pterygoideus anterior or from the reptilian external pterygoid or from both. Their connection with the pterygoideus internus of man would indicate their derivation from the deep portion of the capiti-mandibularis of reptiles. The insertion of the tensor tympani on the handle of the ADAMS, PHYLOGENY OF THE JAW MUSCLES i511 malleus would, on the other hand, indicate relationship with the anterior pterygoid, which in reptiles is inserted on the back part of the mandible. The topographic relations of the tensor palati suggest one of the cranio- pterygoid muscles of reptiles. The tensor veli palatini (tensor palati) is called by Gaupp the homolog of the reptilian “pterygo-mandibularis” (anterior pterygoid). He points out that Kostanecki (1891) in Didel- phys showed the close relation of tensor tympani and tensor veli palati. Killian (1890) also found that the two muscles were joined in the early stages of apes, man, Cheiroptera, etc. In man the two muscles appear at about the fourth month to become separate from the pterygoideus inter- nus. ‘These facts in the embryology would appear to show that tensor tympani and tensor veli palati are phylogenetically related; that they come from the pterygoid musculature of the reptiles, probably from the anterior pterygoid. Their close relations with the mammalian ptery- goideus internus also indicates that they arose from the same region of the temporal mass. This derivation of the tensor tympani from one of the pterygoid muscles seems very plain, for it extends down along the side of the eustachian tube in the mammals and is attached to the handle of the malleus, so that the essential relations are not changed in the transformation of a jaw muscle into an auditory muscle. The mammalian pterygoid muscles may possibly both be derived from the deep part of the capiti-mandibularis mass of reptiles—that is, from the capiti-mandibularis profundus or pterygoideus externus. The changes in the posterior part of the jaw and in the skull would make it almost impossible to derive these from the anterior pterygoid muscles. The diffi- culty in deriving the pterygoid muscles of mammals from the anterior pterygoid muscles of reptiles is that the latter are inserted on the posterior part of the mandible, an element which became vestigial, while the former are inserted wholly on the dentary. A study of the jaws and skull of Cynognathus and the other cynodonts indicates that it would be difficult to conceive clearly the transference of the insertion point of the anterior pterygoid from the angular and prearticular region to the back part of the dentary. On the other hand, the pterygoideus or capiti-mandibularis profundus, which is inserted in the region of the coronoid, is in such a position that the upgrowing, ascending ramus of the dentary might well invade its insertion area. In this way a muscular connection between the wall of the cranium and the inner side of the dentary might easily be established. As the reptilian palate and lower jaw became completely transformed, it is to be expected that the characteristically reptilian pterygoideus anterior would degenerate along with the elements on which it was inserted. From the innervation of the mammalian pterygoid 152 ANNALS NEW YORK ACADEMY OF SCIENCES muscles, which is from different parts of the ramus mandibularis, we may assume perhaps that the pterygoideus internus has long been sepa- rated from the temporal mass, and that the pterygoideus externus is the newer muscle that still bears a close relation to the temporal muscle in the mammals; and so far as I have discovered, the pterygoideus externus is present throughout the Mammalia, while the pterygoideus internus, which has been separated from the mass for a longer time, has in some cases (monotremes) disappeared or never developed. Of the pterygoid externus Lubosch (1913, p. 75) writes: Der Pterygoideus externus hat keinfalls seinen Mutterboden in einem der Pterygoideusteile der Urodelen. Die Darstellung von Schulman, dass er einem tiefen Teile des Temporalis niihestehe, liisst sich aus den Verhiiltnissen der Urodelen verstiindlich machen, wo der Temporalis geradezu iiberraschende Differenzierungen in einzelne Portionen zeigt, so dass sich die Ansicht recht- fertigen liisst, das der Pterygoideus externus in bestimmten Teilen der Tem- poralis-muskulatur der Urodelen enthalten sei. But unless the amphibian ancestry of the Mammalia were accepted, one would question the homology of the mammalian muscles with any- thing found in that group. Of the pterygoideus internus Lubosch (1913, p. 75) writes as follows: Der Pterygoideus internus scheint hingegen alter Besitz der Ahnen der Siiugethiere der sogar seinen Ursprung bewahrt hat. MHinsichtlich seiner In- sertion ist zu errinnern, das schon bei Fischer Ausbreitungen am Dentale bis nach yorn vorkommen (Vetter) Auch er fehlt den Monotremen, so dass hier die gesamte bei Urodelen angelegte tiefe Temporalisschict nicht vorhanden ist, der 38. Ast des Trigeminus also unmittlebar unter der Wangenshleimhaut liegt. Da sich bei Urodelen der Pterygoideus internus aber auf allen Stufen der Emanzipation vom Temporalis zeigt, so lassen sich ausgangszustiinde auch fiir die Muskulatur der Monotremen mit Wahrscheinlichkeit feststellen, ohne dass die Frage hier erdtert werden kann. I certainly agree that there is no relation between the muscle of the urodele and the mammal, except in the general scheme of homology of the temporal mass, but the rest is special development in both groups, especially in the pterygoid musculature. Regarding the digastric, it is now generally agreed that it is not the depressor mandibule of the Reptilia, Aves, or Amphibia, but an offshoot of the same parent mass, namely, the second constrictor, that has been retained throughout the vertebrates and has always supplied the depressor for the mandible, except in the monotremes. ADAMS, PHYLOGENY OF THE JAW MUSCLES 153 GENERAL SUMMARY OF HOMOLOGIES AND EVOLUTION OF THE JAW MUSCLES IN THE VERTEBRATES The following conclusions may be regarded as well established: (1) The two chief muscle masses of the jaw, (a) the adductor mass innervated by the ramus mandibularis (V.,) and (b) the depressor or digastric mass innervated by the facialis (VII), are homologous through- out the Vertebrata. (2) The adductor of the Pisces is the mother mass from which the muscles of mastication in front of the quadrate are derived throughout the vertebrates, by the separation of slips of this muscle and by their gradual complete separation in nerve supply through the growth of the originally small twigs into separate nerve branches. In the evolution from fishes this adductor has changed profoundly and much of it has been lost, but the original topographic relations of the muscle remain the same throughout the vertebrates, whether all parts are completely sepa- rated or not. (3) Some of the so-called pterygoid muscles have been developed inde- pendently in the amphibians, reptiles and mammals. The new develop- ments have been slips needed in the movements of the pterygoid region in streptostylic reptiles, and probably a new series of pterygoid muscles for the mammals. (4) The amphibians retain rudiments of the levator arcus palatini of the fishes, but these muscles are lost beyond this group. (5) In the streptostylic reptiles the complex pterygoid musculature was developed to meet the needs of the movable quadrate. (6) The complicated musculature of the pterygoid in the streptostylic forms which was developed by the subdivision of the capiti-mandibularis mass into numerous slips. (7) The birds inherit much from the reptiles, and parts of their ptery- goid musculature are undoubtedly homologous with those of reptiles, as follows: Pterygoideus profundus of the birds = capiti-mandibularis profundus in the reptiles. Pterygoideus anterior of the birds = muscle of the same name in the reptiles. The capiti-mandibularis superficialis and capiti-mandibularis medius = subdivided capiti-mandibularis of the reptiles. (8) The tensor tympani and the levator veli palati of mammals ap- pear to represent vestiges of the pterygoideus anterior of reptiles. The tensor tympani took on its modern functions when the back part of the 154 ANNALS NEW YORK ACADEMY OF SCIENCES reptihan jaw gave up its primary function and was taken over into the service of the auditory organs. It retains its old relations with its inser- tion on the handle of the malleus (in reptiles it was inserted on the retro- articular process of the mandible and on the prearticular). The levator veli palatini seems to represent a part of the same pterygoideus anterior that has changed its relation shghtly to serve in the region of the mam- malian palate. (9) The region innervated by the facialis has changed somewhat in the mammals. The depressor mandibule of the reptiles has given rise to the m. stapedius of the mammals and the rest of the muscle has dis- appeared. (10) The posterior belly of the digastric of mammals appears to be a new development that has come from the remnants of the posterior part of the second constrictor (C,sd) of reptiles. It is thus believed to be a new muscle which has nothing to do with the “Digastric” of the reptiles, although it has the same functions and the same innervation. To this posterior belly has been added or grafted on at the anterior end one of the long ventral V, muscles to make the anterior belly of the mammalian digrastic. (11) The slip that functions as the m. stapedius in mammals appears to have started in the reptiles and has been retained from the depressor mandibule. This muscle, which is connected with the distal end of the stapes In mammals, started in the reptiles and reached its perfected con- dition when it was drawn into the middle ear in the course of the trans- formation of the mammal-like reptiles into the true mammals. As men- tioned above in the sections on birds and reptiles, the future stapedial muscle was in the right position in the reptiles, where the depressor touches the distal end of the stapes, so it is not difficult to imagine how the depressor gave off a shp that became the future stapedius muscle. RELATIONS OF THE JAW-MUSCLES TO THE TEMPORAL FENESTRE oF REPTILES The following from Gregory and Adams (1915) summarizes their observations on the relations of the jaw-muscles to the temporal fenestre : (1) That in primitive vertebrates the chief temporal muscle-mass (adductor mandibule of sharks) was originally covered by the dermal, temporal skull-roof. (2) That in modernized Amphibia and Reptilia, as well as in Aves and Mammalia, one or more slips of the primitive adductor mass had secured additional room for expansion by perforating the temporal roof ADAMS, PHYLOGENY OF THE JAW MUSCLES 155 either at the top or at the sides or in both regions at once; much as in histricomorph rodents, a slip of the masseter has invaded the region of the infraorbital foramen, so that it now extends through a widely open arcade and finds room for expansion on the side of the face. (3) A comparative study of the skull of Tyrannosaurus led to the suspicion that the pre-orbital fenestra of the dinosaurs, phytosaurs, pte- rosaurs, etc., were also functionally connected with the muscles of masti- cation ; but it was realized that proof of this view required a wider study of the jaw-muscles of living reptiles. It was afterwards found that Dollo (1884) had suggested that the pre-orbital fenestrae of extinct reptiles were filled by the pterygoid muscles. (4) The inferred conditions of the jaw ‘musculature of Cynognathus are in harmony with the view that in the mammal the back part of the reptilian jaw became transformed into the accessory auditory ossicles. (5) As a working hypothesis, it is assumed that the transformation of certain elements in the temporal and occipital regions of early Tetra- poda was partly conditioned by the stresses induced upon the skull-roof by the jaw and neck muscles. Comparison with lizards, Sphenodon, etc., clearly indicates that the prolongation of the parietal into the postero- external process, joining the true squamosal, was correlated with the squeezing effect of the capiti-mandibularis and depressor mandibulie muscles. This may also be responsible for the suppression of the supra- temporal and survival of the squamosal in early reptiles. 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Beschreibung des Muskelsystems eines Python bivitatus. Arch. f. Anat. u. Physiol., pp. 346, 482, 528. ALLEN, H. 1880. On the temporal and masseter muscles of mammals. Proc. Acad. Nat. Sci. Phila., XXXII, pp. 385-396. ALLIS, EH. PHELPS. 1897. The cranial muscles and cranial and first spinal nerves in Amia calva. Jour. Morph. (8), XII, pp. 487-808. BARDELEBEN, KARL VON. 1903. Muskelsystem und Mechanik. Ergebnisse d. Anat. u. Hnt. Merkel u. Bonnet., XIII, pp. 114-164. BIJVOET, W. F. 1908. Zur vergleichenden Morphologie des Musculus digastricus mandib- ule bei den Siugethieren. Zeitschr. f. Morph. u. Anthr., XI, pp. 249-316. BRADLEY, CHARNOCK O. 1903. The muscles of mastication and movement of the skull in Lacer- tilia. Zool. Jahrb. Abth. f. Anat. u. Ont., XVIII, Heft 4, pp. 475-486. Broom, R. 1910. A comparison of the Permian reptiles of North America with those of South Africa. Bull. Amer. Mus. Nat. Hist., XXVIII, pp. 197-234. 1911. On the structure of the skull in cynodont reptiles. Proc. Zodl. Soc. London, pp. 893-925. 1918. Notice of some new south african fossil amphibians and reptiles. Ann. So. Afr. Mus., VII, pp. 270-278. 1915a, On the Gorgonopsia, a suborder of the mammal-like reptiles. Proce. Zool. Soc. London, pp. 225-230. 19156. On the South African pseudosuchian EHuparkeria and allied genera. Proc. Zo6l. Soe. London, pp. 619-633. CHAINE, J. 1914. Le Digastrique. Jour. de ]’Anat. et Physiol., L, pp. 248-819, 393-417. Corps, E. 1904. Beitriige zur Lehre vom Kopfnervensystem der Vogel. Anat. Hefte, XXVI, pp. 49-100. 1910. Zur Morphologie des Gaumensegels. Anat. Anz., XXXVII, pp. 305-318. CUNNINGHAM, D. J. 1891. The value of nerve-supply in the determination of muscular homol- ogies and anomalies. Jour. Anat. and Physiol., XXV, pp. 31-40. 1902. Text-book of anatomy. New York. DANFORTH, C. H. 1918. The myology of Polyodon. Jour. Morph., XXIV, pp. 107-146. ADAMS, PHYLOGENY OF THE JAW MUSCLES 161 DEAN, BASHFORD. 1909. Studies on fossil fishes (sharks, chimzeroids and arthrodires). Mem. Amer. Mus. Nat. Hist., IX, Part 5, pp. 211-287. Dogson, G. E. 1882. On the digastric muscle, its modifications and functions. Trans. «Linn. Soc. London, (2), II, pp. 259-264. DOoLtLo, L. 1884. Cinquiéme note sur les Dinosauriens de Bernissart. Bull. Mus. Roy. de Belgique, III, pp. 129-146. DRUNER, L. 1903. Uber die Muskulatur des Visceralskelettes der Urodelen. Anat. Anz., XXIII, pp. 545-571. 1904. Studien zur Anatomie der Zungenbein-, Kiemenbogen- und Kehl- kopfmuskeln der Urodelen. Zool. Jahrb. Abth. f. Anat. u. Ont., XIX, Heft 3, pp. 4385-622. BHASTMAN, C. R. 1906. Structure and relations of Mylostoma. Bull. Comp. Zo6l. Harvard College. No. 1, pp. 1-29. 1906a. Dipnoan affinities of arthhrodires. Amer. Jour. Sci., XXI, pp. 131-143. Ecker, A., and WIEDERSHEIM, R. 1896. Anatomie des Frosches. 3 Auflage. Braunschweig. EDGEWorRTH, F. H. 1903. The development of the head-muscles in Secyllium canicula. Jour. Anat. and Physiol., XX XVII, pp. 73-88. 1907. The development of the head-muscles of Gallus domesticus, and the morphology of the head-muscles in the Sauropsida. Quart. Jour. Mic. Sci., LI, pp. 511-556. 1911. On the morphology of the cranial muscles in some vertebrates. Quart. Jour. Mic. Sci., LVI, pp. 167-316. Fick, R. 1904. Handbuch der Anatomie und Mechanik der Gelenke Thiel I-III. 1904-1911. Jena. FURBRINGER, MAX. 1897. Uber die spino-occipital Nerven der Selachier und Holocephalen und ihre vergleichende Morphologie. Festschr. f. Carl Gegen- baur, pp. 351-788. FUTAMURA, R. 1906. itiber die Entwickelung der Facialismuskulatur des Menchen. Anat. Hefte, XXX, Heft 2, pp. 485-514. 1907. Beitriige zur vergleichenden Entwickelungsgeschicte der Facialis- muskulatur. Anat. Heft, XXXII, Heft 3, pp. 479-575. GAUPP, E. 1905. Neue Deutung auf dem Gebeite der Lehre von Situgethierschiidel. Anat. Anz., XXVII, pp. 273-310. 1905a. Die Nicht-homologie des Unterkiefers in der Wirbeltierreihe. Ver handl. der Anat. Gesell., Versammlung 19, pp. 125-140. 162 ANNALS NEW YORK ACADEMY OF SCIENCES 1911. Beitriige zur Kenntnis des Unterkiefers der Wirbeltiere. I-III. Anat. Anz., XXXIX, pp. 97-135, 433-473, 609, 666. 1913. Die Reichertsche Theorie. (Hammer-, Amboss- und Kieferfrage.) Archiv fur Anat. u. Phys. Jahrgang 1912. Anat. Abth. (Sup. Band). GEGENBAUR, C. 1898. Vergleichende Anatomie der Wirbelthiere. I. Leipzig. GoopricH, E. SN. 1909. ss. cies cles steric eeisres eherare IDEade Ss Levator operculi......... i ARR ora SS CO I, L. o. MA@SSELEE ss 24 60 e IRN aay ee sett Mas. IM ylohyOld sich. ce Sie Sete oledaluess ele sFahNetowricicrsy crete che yg Mayenne Parieto-mandibularis...... Sos wee es PORES focal Pa. m. Protractor-hyomandibularis.......... MOE ODI nO Oe BP hy: Pterygoideus anteriomar-crtcris eictsicve er ete cievet reeds Ptant: Pterygoideus externus...... RDO ere Of So unwe delis eaxit Pterycoideus WMtErMUS ee. eye ote les eis = Bieta eVenabete Rictek hohe ea le Goer Pterygoideus posterior...... ageyeretere Sister Shs vatteceus calfohehay ale, sine Pt. post. PteryZoldeusSiparietalisc..ccyt emrer terete Nelete ctekener eterna Pt. pa. Pteryso-sphenoidalist.. cise ontetamete cnet Saad oooa aay Jem iS}Nl ae Pterygo-spinosus............ aiS°d lane de Yoregate VeRO peoleta mer Saale ess 124 Sh Pteryeo-tyMpPaniGs, 2... Aaere caieetevs ove Sear tere Po oo SiS ae Retractor byomandibularis...........- hacdodoupameod laa liny Spiracular muscles. .cs.w clack aieyaleteteL peveliotav snare sie een OTE StYV]TOUGEUS : 05 eels aie Se onele o's ee O Saree nen aerate te eraietotars Ghee St. PemiPOralisin rec «el lacke = dj cicta) onetetemmicicceteraterers ara cle sence Tem. AY ZOMACLCo-MAandiPUlaris. caves oe criccnctetetererstonslertereral ketone any nae PXPLANATION OF PLATE I Jaw muscles of Scylliwm and Acanthias, representing the Elasmobranchii, and of Acipenser and Polyodon, representing the Chondrostei Fie. 1.—Scyllium (Mustelus). (Muscles mainly after Marion.) The adductor mandibule (Adm.) is seen wrapped around the posterior ends of the palatoquadrate and Meckelian cartilages. It is not differentiated into separate parts, although the fibers are crossed in certain regions. ‘The levator maxillz superioris(Z. m. s.)and the first dorsal superficial constrictor (C. ,sd.) lie in front of the spiracle and together with the adductor are innervated by the third branch of the fifth cranial nerve (V,). All the pre-spiraculiar (pre trematic) jaw muscles of higher vertebrates are derived from this group. Fic. 2.—Acanthias. (Muscles mainly after Marion. ) The muscles of the visceral arches may be divided into two sets according as they are derived from muscles which originally ran in a transverse (cir- cular) or in a longitudinal directicn (Kingsley). The jaw muscles belong to the circular group, which comprise dorsal and ventral series of constrictors and the adductors of the visceral arches. The levator labii superioris (JZ. 1. s.), levator maxille superioris (L.m.s8.) belong to the series of superficial dorsal constrictors (C. ,sd., C. .sd., ete.). The adductor mandibule (A4Adm.) belongs to the adductor series. Fic. 3.—Acipenser. (Muscles partly after Vetter.) The adductor mandibule is small. The levator maxillwe superioris is either absent or not differentiated from the large protractor hyomandibularis (P.hy.). This represents C.,sd. of the elasmobranchs. It lies in front of both the spiracle and the hyomandibular and is innervated by V.. The retractor hyomandibularis (FR. hy.) and the levator opereuli (Z. 0.) represent C..sd. and are innervated by VII. Fic. 4.—Polyodon. (Muscles partly after Danforth.) The adductor mandibule is divided into two muscles (Adm', Adm’). The post-spiracular constrictor (innervated by VII) is more or less subdivided into retractor hyomandibularis plus operculi (R. hy. + 0.), levator operculi (ZL. 0.) and adductor opereculi (A. 0.). seat danettyd braaehielis ae é , =r. ake ote eect ra" we Ra a A atiictx f, opted yg Tae! sAd * 4% ne “Fs 2 + Auctar ena Je Eevurers pcp ems ae pierre sole a vey lap ne “motea : . if mus ie mare f hy i ifs 74 £) metre enperk ts ; aPly twhraiaog, 913 Watorns Dogar ys otat botntiayi Rib, gon lL. ask bats k wolerol oth Wepolesrt wikis ai he PAAYLD OU (i Mgcodites ( Ane, SO )atoftayos isiaque leewh Jet ods heen (4 we hai at bos rust ote sretotshiyg elt 1 ive ositonod ber eri to don ath) rato p tie od AIA LT coxa kates “tit otf). to dot ; nog, Alf i zaigidas19y tostghit ‘to sofseann ‘ne tA + SORRENTO caer; tai ubaat eotvath) — aviks SeehtO9 9K ioe "yu ge oti babiy ‘ iy) serryaiue ‘S iit fist itn ah 9% ak yitolsd eotseisun Wai, a “Cis toset 5 ‘ore ‘caliecat rata AIO lotitees 1 Poltge inilase’ bag lakiof seitqaiod doin quote FSS Ate § B) EPotio oti. Hdet 16 a os tee rr brs ate 9 drs ies fnertoh. tsisthionr ia i hee { i rat ee i BP “oS 3" Sbevatpt apercail s (ietisT: sattn iieine zalyes) Gea orn Be rane ‘godtio: ot SMOLSYBS silftzner toLaval-ofT: “llettd a atedthoped ToREDbI as rhiludibinigent tolsrion sytal ‘alt: nowt: hosuiiaaiattify’ Jou? 46 diod to tout HE BIE pdiitrdodrents, aft 30.2, 2° ahaeonyos aT yo RG aT: de yt pelivalt er how: ialodibinmoyd ott Dire of Hite bes asa CP LY Stitrraqo aojnvet die thong ¢ ie on 4 Bis {ida pet apesr: VOL OAM ay ee ye FLY “ut holeesonts Poorvaronth Prien ete ns eo & tee os B tiie etey tes ‘eatachanel asia. ltenig ial abo d ott .CoiéYibAmabanttieid ojai bobhib- ai wtudibasur ri oft es ‘olat bobivihdazaeob te oom at (EV. yd botayrenai) sojortiaces natbolsioe oie GP, (io .c) eee ae ne Ay ee aitelsdibasanl Yd rotgettot ~ be a Weryenty imp ar wakes .> wan pia aia Peg hot aes favtSartor strane ib (uaa. ee assis ast Ape riaris eS > eS Doh g qihceL aie ae aokrge Rriresiiak Mmuseiesd gc i SEVER Ves Seo eS ag Sa tee ak WietHtie. - Sr cumn tile-Mala urinates . at i ANNALS N. Y¥. Acap. Scr. VOLUN XXVIII, Prate I Lims. C.4sd Cosd C.3sd Casd Crsd Soo bates ao (Cifsd IQXPLANATION OF PLATE II Jaw muscles of Amia and Lepidosteus, representing the Ganoidei Holostei, and of Anguilia, representing the ‘Teleostei Apodes Fires. 1 and 2.—Amia. (Muscles mainly after Allis.) In the Holostei and Teleostei the adductor mandibulize and levator maxillze superioris of selachians are subdivided into numerous muscles all lying in front of the hyomandibular and innervated by V,; while the C..sd. group behind the hyomandibular, which is innervated by VII, includes three muscles (Ad. hy., A. 0., L. 0.). The levator arcus palatini (Z. a p.) and the dilator operculi (D. 6.) belong to the C.,sd. series and appear to represent the pro- tractor hyomandibularis of the sturgeon (PI. I, Fig. 3). The levator maxillie superioris (4.m. 8s.) includes several slips of which only one is shown. The first division of the adductor mandibule (Adm,) covers the cheek behind the eye and runs from the preoperculum to the mandible. The fourth division (Adm,) is lodged in the Meckelian fossa of the mandible: The dilator opereculi (D.0.) passes through a groove or depression in the hyomandibular and is inserted by tendon into the operculum. Fie. 3.—Lepidostcus. In correlation with the anteroposterior elongation of the head the jaw muscles are nearly horizontal instead of vertical. This oblique insertion gives great speed but low power in the closing of the jaw. The protractor hyomandib- ularis (P. hy.), as in Acipenser (Plate I, Fig. 3), runs from behind the eye to the anterior border of the hyomandibular. .The second slip of the adductor is now above the eye instead of behind it. Fies. 4 and 5.—Anguilla sp. In this peculiar teleost the adductor mandibule is greatly enlarged and has spread out upon the top of the skull after the fashion of the capiti-mandib- ularis of certain, Amphibia. The large dilator (D.0.) and levator operculi (ZL. 0.) muscles assist in the strong respiratory movements of the branchial region. — ~ frie intent 4 i gnitieed es Rucgees —— “es sditx ear’ sidered frets: snared Doiusteek “ni yrigt fin- voloens on ZO UNE be quote Bes set atid 2 at Het errocadt ‘iss zoloznat serdt} zobriont IPF yh botarrasat et dstity ¢ sodnlif si} bite (qa A) int aloe, B91 solmevol po arn “ong oft judestget of TRIGCE ban asi .be, oft of Sotecd eli Lc pest yoteval oft Ce Tas eskh) foggtirie ods to its oft vole ei s00 y lie ioidye to agile Ie19v92. ‘gobutont Ait X) a. Sotislod oseiae af e19r09 Gay pipe ms kal eae 0 at avr intaibraumered ould ni woizaeutob 40 Frou date oF ra am Preniasens? oe in a x " ‘2 - f Wht oiit baat: ont to camachots Sols ameabtin aiff rr 5 is entkg oitioedt supilde aid —[goitt97 To: beotent {giaosito. er ets dihasero yd soMetiorg odT swat off to. zitlaals ol} wk soon wor Ind 44 of 474 edt baided suort ace .(& 2tT 1 otelL) woewsgiok ui zi-oionbbe oil? to pate Bit0992- ‘ont, errr weep oul 40 * oe — A _ sad boas bouvalus. ylinore ab a eaniebiber sotouhipis ants j20910% * : . dihirioriigns off to soident oft 19392 Ile ot to qoi-oilt MOT Husreq9 totaal bas cS A) totstth ogra er “sabditqa ds ainit99. oes acc se a eee. ¢ ~ ANNALS N. Y. Acap. Sct. VoLUME XXVIII, PLATE II EXPLANATION OF PLATE III. Jaw muscles of Hsoxr, representing the Teleostei Haplomi, and of Palinurichthys, representing the Teleostei Acanthopterygii Fries. 1 and 2.—#sov. (Muscles mainly after Vetter.) The superficial muscles are shown in Fig. 1, and the deep muscles in Fig. 2. In this rather primitive teleost the divisions of the adductor are substan- tially the same as in Amia (PI. II, Figs. 1,2). The protractor hyomandibu- laris is either absent or not separate from the levator arcus palatini (Ll. a. p.). The fourth branch of the adductor fills the Meckelian fossa of the mandible. The dilator operculi (D.0.) passes above the hyomandibular and the pre- operculum in the normal manner. Fries. 3—5.-—Palinurichthys. (Identification of muscles after Vetter in Percd.) These are the jaw muscles of a highly evolved teleost. The protractor hyomandibularis (P.hy.) is distinct from the well developed levator arcus palatini (Z.a.p.). The retractor hyomandibularis (Fk. hy.), the levator oper- culi (i. 0.) and the so-called trapezius of the pectoral girdle are extended for- ward above the orbit, on either side of the sagittal crest. abhe ATA, cae 0 oun nssx | oy ‘bee “mnogo iataoalaT orf sunicneiaad sament’ wr voleascn ents . is sisi Suan iatzoolott a aabipacs saat penne: ; holies Ane aot i “alain ollinisbt. ) soe ie ban ts , sin ug Batons qoob ot bur .f Cait af mods STG zat vastett Init at x pavers 916 totubbs ot to anoiaivib oft teoolot orritianieg soi eid) oT £4 “ndibunao ct wodyetitioig att. OU sesrd ar IE) dL. ni aft ante ont lal ard GS A) Lud el ase arts sornval 5itt tort odin FoI 10 due oad todtio et atte Pore aldihanar ot Ato gaz ‘isilodyoMe ot _aiLit roDn hh oft to done irTirot 94 od off Din. aBledibas actord iL outs arodhs - ORR « (.0) GQ) “itrss9a0. rolality’ % he oe navy ir 49194 wort & eotoercet to anitiensn90) 10t9R bate 9dt- ; “soqo ines ait. ( ae ‘Ly Bits aditecadnioed tsar ‘ott wa aD al) fal “wot hoprrstxa ois ‘afbiis Lietotssg 9di to anixaqest hollso-07 ot “ban As A) IL : i 429) (nithene alt to abia totttie fo tiatxo axtt Yovoda i “ANNALS N. Y. AcAp. Scr. VOLUME XXVIII, Pate Ill a 7 iY 5 i = ’ aoe id eo. eo) : es EXPLANATION OF PLATE IV Jaw muscles of Polypterus, representing the Crossopterygii, and of Neoceratodus, representing the Dipnoi Fies. 1 and 2.—Polypterus. (Muscles mainly after Pollard.) The deep muscles are shown in Fig. 1, after the removal of the preoper- culum, and the superficial muscles in Fig. 2. The superficial layer of the adductor mandibule (Adm.*) extends from the enlarged hyomandibular to the ascending ramus of the mandible. The deep branches (Adm.*, Adm.*) run nearly at right angles to the outer branch and arise from the side of the skull. This arrangement foreshadows the differen- tiation of the jaw muscles in amphibians into masseter, temporal and ptery- goid branches (Pollard). The levator maxille superioris (Z.m.s.) is closely associated with the protractor hyomandibularis (P. hy.) as in elasmobranchs. A small slip of the latter muscle runs dorsad to the spiracular ossicle. The dilator opereuli (D.0.) seems to be absent. In Fig. 1 the hyomandibular is seen with the area of origin of the adductor mandibul on its anterior border. The levator operculi (L.0.) is closely associated with the adductor hyoman- dibularis (Ad. hy.). i Fie. 3.—Neoceratodus. Side view of jaw muscles. The superficial layer of the adductor is fastened posteriorly to the opercular region. The middle layer (Adm.*) runs over the top of the massive chon- drocranium and separates the dermal skull roof from it. This part of the adductor is divided into numerous small fascicles interspersed with connective tissue. Fic. 4.—Neoceratodus. Viewed from above. wt meat hed rowan QA duis ee. Sea a m bse “kayraiqozeo19 oa ghtiebsiae seme 3s aieken ook me | ional out ec aicat ena fe ae. = © aia ni eatocilin Isivitroque oi ae acer ee acts most ebaotiza (- wet} “aludibunt Toisubbs 93 to 19461 Ininitroque ott qo9h off ofdibasac oni Yo eume1 zaibascae 9di of tetedibasmoyd posts rs bas doasid toiso ot of calgon iizit ja yutinon nor (sw ¢ wath BL) zooanid oe, -11attih aft awobsdeosot inomegariis aidT lsd adi to. abiz ont at + ouits. = Lisk hug {exoqest otezennr ofa eusididqern ai 2ofoe iat wah alt To solsit be ; ylazols ai (2 AW aL) aitoiteque slixsm totsval ait .Crretlod), onountd bioz | ; ado eidomants i 2s Cyt ak} aireludiba gato sotsetiong: ott dit botwinozen ; od ofpiaee istyoerige oii of} Deetobh anu. obverse yates ont Yo qite sare A ei teludibusmoyt ody Cyt'L al duseds od oF amooe (.6 A) Husasqo soleil: e . stebted toieias ali co elidibasa totubhs dd} Yo aizito. to OCR oily itw 1 199% “igmoyd soloubbs ody Atty hotrinogz& ylozolo ai (.0 1) ifnteqo 10879 oft +f - aN g th Coat Mh) shat es ole iit wat to weit ohia awboinvaanat— a ork selatogo oii o1 Yroiiatzoq hoasiant ai sojonbba eit to to ysl [nioiroqire ott £ aos ovieaser ot to qot edt to7o ano Caebh) tovel of bbint oT “coret 45 of} to tieq eidT ti uot toot finda [gortoh orft Poth Bq94 bus rt tt ee avitoonitos mtiw hostoqa19ist aatoinen't ieee HO TOME otai hobivily 2i sotoubie # $23 Te ae, Sh outed artsy, ar . : 5 ‘ ~ : - het ae F itieeek aims ; sods moet Howart ; ue F ANNALS Ni ¥. ACAD: Scr. VOLUME XXVIII, Puate IV EXPLANATION OF PLATE V Jaw muscles of dana, representing the Anura, and of Cryptobranchus japon- icus, Amphiuma and Amblystoma, representing the Urodela In all modern Amphibia, owing to the loss of most of the derm bones cover- ing the occiput and temporal region, the jaw muscles lie immediately beneath the skin and extend above the parietals. The muscles are divided into a pre- trematic group, in front of the squamosal and quadrate, innervated by V;, and a posttrematic group behind the squamosal and quadrate, innervated by VII. Ites. 1 and 2.—Rana. (Muscles after Ecker and Wiedersheim. ) The superficial muscles after the removal of the maxilla are shown in Fig. 1, and the deep muscles in Fig. 2. The adductor mass of fishes is now represented by the capiti-mandibularis (C.m.), which is divided into anterior and posterior slips running respectively from the squamosal and quadrate to the mandible. The deep part runs from ithe under side of the squamosal to the mandible. The pterygoideus anterior (Pt. ant.) probably represents the deepest part of the adductor mass. The depressor mandibule (D.m.) lies entirely behind the otic region and is inner- vated by the 7th nerve; it may have been derived from the retractor hyoman- dibularis of fish (Pollard). Fic. 3.—Cryptobranchus japonicus. (Muscles mainly after Humphrey.) The deep anterior part of the adductor mass of fish is here represented by the much-enlarged pterygoideus anterior, the outer part by the capiti-man- dibularis superficialis (C.m.s8.), while the levator maxille superioris may be represented by the capiti-mandibularis profundus (C.m.p.) and pterygoideus posterior. The depressor mandibule is divided into two parts, of which the anterior is large. Fie. 4.—The same seen from above. Fic. 5.—Amphiuma. (Muscles mainly after Driiner.) Fias. 6 and 7.—Amblystoma. (Muscles mainly after Driiner.) The superficial and middle layers of the jaw muscles are seen in Fig. 6, and the deep muscles after the reflection of the capiti-mandibularis in Fig. 7. 7% / . ‘ 7 Ue sad ; ( 4 : ; * a x ie oT _ . hs = ; be eee te i : ou Sala ‘ae ee & iP ee er, eee ek Vo avast 40 ie acca AG iy Se ( i ‘ ‘ ts a Peo Near is i eat onal oe fen Ae ieee ie soot avrtoerdorqun’ to bas rd of sabitoronqor SL to ee ; aie “oi Bloboxld ont galiaseosgoy sbawobngh ath Na mac Ke ao ’ ha 1909 Sac een odt to jeomt ‘to aaol ate, ot wutwo sidikiqeuy? ree he “iitizonod ylotsibomuart 9il voloan at wast oft .foinet Teroqurs pas juqins0- OT & olud hebivib ots ealenan oft Pigieis sy ott sTods baatze Dut 3 hie .¥ yd beievrousal sieibagp: fous [Reocnsn pe oly ‘to Jott sti (OTE + ahs Lis tS d bein terbsesp bas freomt amps oy Ahoidod gor cnet (.1oiedetohoiW- bus yadocL sorte zoloaul) Seo & Dare I 20" fait ceworle TS siliznat otht to [eromot off tots 2ol yes Iaisitsoqua oAT nD ca 8 mee ‘Pode = -3tT i 2ofoanat qoot ott ban - giteludibasar- Diqss ot yd ‘hsiaszoiqot wou ai zoel to east sotoubhs od ¥ ee Yisvitesqest pevieeetirs aqila sorieiz0q bin tortetas ojai hohivib vt deobetyy (00 Kes ' stort curt uy qooh oF .ofdibtrsar oft oF SIn1besp bas [n200i6ipe adt 0071 “A rors eusbiogyisig oT | .oldibasn of) ot ineonmisope oft to shia Tobias onl ofT .2e2nnt tojonbbs 94d Yo Tsq Jzoqoop oft ago (St uldadoty (S90 wy _-“enoi ai bias aoiget oile oft Daided yloriine Zoi (6 AL) esindibasat rozaatab: ese : nent sotonniat 9d mott boviteb n99d oven Yam ti ;ovred MIT Od BAU bots? oe Roe ok “. 5 ite (bralloT) ait to pas se 7 y : = = ve qodquacH nothe qihisios zaloairM) eo" Secodne Pat saMWorqyed—e ork EG yd boinsesiqer oted 2i dail io e4nor oto ROR oilt to. Jisq toheias qoob: oT : o -1sM-igek\s ot yd teq ‘totvo off LorTotiHs anabio: Siggy host slis- dover sie 4d Ys0n zivotiequa sllizsin tossvel oii ofitw , 0.8% 10) ailsindwoque elisiudily Z ; c " <4 -- * anrabiogyisig bas (. i k Y). anbintorg zitsindibasm-iiqes. oft “ed Deinsesrgyt | S fo eh ie doi " to avsea owt oigi bobivify i olodibtnce soseorgesh od toiadezoq — ee a pry ‘3 sarsl al eis _ Yi yy * . plo i, . ; 700k Mort 199% 90K od a yy atts ( ont “tote utisont zofoanl )- oohrenance jovi cs x ; ; OS a a Cent ' (renin * tts whatnot aot: yah) sand Sughd see —.7 bas a. Bord ; Teg lea tat bas 28 ofa of 1992 916 zoloaaran: wisi, a0 to atest sfhbiar bas (sisiisqna oat be at Lae 7 .3iH ni eitelodibienm-tiqes oft To moitsoAher ait noite volorasstr qeeb. ont z < Ns : Ss 7 : - : : x bey ~~ if ANNALS N. Y. ACAD. Sct. VOLUME XXVIII, Puate V C.m.p. Pt-ant C.m.p. . +tbire allt Jo juort mt as9a 918 Gt ud, hotsvsouni) nota oi 8 eee m9 oie (ITV ad cheat: At Si GUNN ITZOK att Brees Pa ea is ‘ ms < are at TRL iq od} aan e aft wi ese on ‘pofoauar ip of: Dihas, 1004 finde fearrb ott betsitonsg aed Cs OPS eiueindibmanat itiqa? oft ees. ete Tpitiane zuol oii of bedoktE 2i guineqo feroquastizng ont dyson Biniee sols rae 3 3 con = aitnindibuge-iigss odt aos" qlonpilde east sortahan ajobiowy raid ott - Heat ia ’ oT ri, off zaivoly at oisrayioy eolvanat o7rt “yaotT = { AAV WA, Ye ae 9) im ae, ‘Tazouignpe ot to wbt0ed tobiaizeg out usort eszitn ( sit i) eigdibasar “ozzonob , ork to aaosong tobisizog ot ofai botiseit ei bee noizot yiotibos elt toaidtodd -0722_ 2 ' atttolsd® att ui ao fein ee nies att ‘to tae omrarttze ott steibinnt a ee a gu ReO; 19T1K wiatean) wobions hg ba & ee ~ , a Bb il ui eoleunt good) i bis .& .2iL ai awods ons eofoerint Isisitvoque of _ attstodifusariiiqns oft Yo soz! (s9toe2hon) Ieinftiaque att To eordit oT '. .. ofbbiar od 2z9lgan jdgit te yiirom teyel sibbiar of 16 seodt 220m (8 We 9) ~ adT .nitzonet Intoqamisique ost to arobi0d alt Panots 2o2h1e (16 A Dy) Secetie + = otT: ofoatrar 2idt to noizspdz9 oft io? s9nge ebiotts sttasitet Esto): Inreodnl— “SS | - frioqioisiggse sy to z1olytod sovidl ot mott 292ite (.q 10 Y) 19a qos aft .ofzer Yrolibun oft ai 5en9-1i Bd ad? to sbiz odt- tort boo srieoast “* Wigs stab 10d upiaeog yorwol oft bavote bolroesti 2i (. Jan J) soroiae ausbiog tan ae Ll i an ; : 5 ’ ee dibasase ot to : soni h- a) ite G ath ~ gpiioadvolt ott ods haben AE (AD A “y 7-8 SO) erialodibunueitiqss ait [sioqaatriqne ei, oft ot qu eaot qile ofbbine oP di bisa oils to naa “ Drored -Digwtet bobneizs at (.\we A) toretir abobionytaty oT . TNT Dirtrors AIBN, (nog AU} 407 4o)2oq evohiog vrai oft Jide off Tor tH07% 4 Dar ie : = sidibasar alt to i page ph att 7 ANNALS N. y. ACAD. Scr. VOLUME XXVIII, Puarr VI EXPLANATION OF PLATE VII Jaw muscles of streptostylic reptiles: Jguana and Varanus Fics. 1 and 2.—/guana. (Muscles mainly after Mivart. ) The superficial muscles are shown in Fig. 1, and the deep muscles in Fig. 2. The capiti-mandibularis is divided into three layers. which have the same arrangement as in Sphenodon. The pterygoideus anterior (Pt. ant.) differs from that of Alligator in being confined to the posterior part of the skull. Fies. 3-5.—Vardanus. (Muscles mainly after Watkinson. ) The superficial muscles are shown in Fig. 5, the middle layers in Fig. 4, and the deep layers in Fig. 5. In correlation with the extreme mobility of the jaws the pterygoid muscles are differentiated into four distinct elements, pterygoideus anterior (Pt. ait.), pterygoideus posterior (Pt. post.), pterygo-sphenoidalis (Pt. s.), and pterygo- parietalis (Pt. pa.). These are probably all derived from the primitive ad- ductor mass and along with the three divisions of the capiti-zmandibularis are ‘innervated by V3. ss . 3 ~ > oe Ce ae PSs ER RAR Yo 5 “ome ot ored, doiiby. »t9x6l ootdt oti Dabivib at abst 5 eth Cs AN) tobronnis. anabioy rot ott wohasrashgy Se Anil uf 20 tq sotToseod att oF Doaiaoe gato. ot ot we Bgl 4) aerate softs elnisat isin) batt wi zon olbbiat ot a it ite rnonte 916 eoloest oat Pile | oy BL ut: BUTTE ftom low eid apiece Silo. spittdous piuettx9 oct stir a » eC tot) TORTS ‘puobios TI9K | Alnsnrel9 toni quot ini -Hbotet ORL bas -.(.8 A), ailsbionsilea- on ytioky if Seou way? “oltaieod. | af ; o-Dug evtitinriag ott mort beviveh tls “eld sdoniy oi seo og 31), fen Os Fi a hanes oat Yo ae sa Zoe, at a! Das : ke "Sa ths 5 aN 4 a : aes ns 2 | ~- ‘2 * ’ < - , ~ = ¥ i - x! . Ys = iT + fe e ‘m * ts = } > * > -_ a, oe \ ~ ~ ye : as a - J nee ! : a . z. \ , -t ss ‘ i oY # ak 2 e a » t » > « A, .: > ; ; \ i ‘ F ’ : * Pe eet 2 = i % ° < : , EXPLANATION OF PLATE VIII : Jaw muscles of a typical bird: Gallus Fic. 1—Gallus. Superficial muscles. The capiti-mandibularis is divided into a superficial layer, analogous with the masseter of mammals, and a deep layer, analogous with the temporalis. The capitizmandibularis superficialis (C.m.s.) is attached to the quadrate. lie. 2.— Gallus. Deep muscles after removal of the capitimandibularis superficialis et medius. The pterygoideus anterior (Pt. ant.) has the normal reptilian relations. The pterygoideus posterior (Pt. post.) perhaps is attached to the quadrate. The depressor mandibulze (D.m.) is of normal reptilian type. Fie 3.—Gallus. Deep muscles as seen from below. The entotympanicus (Hn. ty.) muscle is 2 remnant of the adductor mass of lower vertebrates. The parieto-mandib- ulavis (Pa.a.) seems to represent a part of the depressor mandibulie of reptiles. ioe haste ; oS a avea'd . conve. nix tPA eee : hy . 7 RD Beaded ‘teenies to ota wih wee aC oe « ae Orns Hoek volvensit: Inbon signe it . a * ae ry Berd ce ae on. euwoxobn ea ed lion 5 osnti Roe te al i vit aludtbireue-Higss eat » otethamp ott ot te if che BF (i % set (Dy ‘llsiott ge fudbawa ig its, Stil oe rn Pans Rh aN es ari an Eee an ait —£ 0 f autbout dy ailaiviteque 4 calydibansn- Hien ont Io, istomet tote ¢ noisier mriliqet Taortoir ‘odt apt) (Me aM). sortaine ssobiog aig pa ' ade aiae ae ‘ot ot bodoeiis ei aqniliog (saog 3V) “tor 19120, enobione® iq oD st : oa. fe ilitye's Inarion to ai (9.44) sols eee 9) re a w-¥ Saubicaeas ae ott s “to sabi bergen SPE of ott to Yreg B usenet 01 aiteooe ( We, ee “eet i ‘ fh o pa a d ANNALS N. Y. ACAD. Scr. VoLUuME XXVIII, Pratr VIII Pt.Post EXPLANATION OF PLATE IX Jaw muscles of monotremes: Ornithorhynchus, Echidna. Atter Schulman The jaw muscles of monotremes as well as their skulls exhibit peculiar specializations along with certain very ancient characters not found in other mammals. lic. 1.—Ornithorhynchus. Superficial muscles. (Muscles mainly after Schul- man.) The capiti-mandibularis of reptiles is now represented by the masseter (Jlas.), temporalis (Tem.), and pterygoideus externus. The masseter is sub- divided into several muscles of which the detrahens mandibule (Det. m.) simulates the depressor mandibule or reptiles, but is innervated by the man- dibular nerve and not by the facialis. The digastric is not present as such. Fic. 2.—Ornithorhynchus. Deep muscles. (Muscles mainly after Schulman.) The anterior end of the temporalis (7Vem.) is attached to a strong ligament running from the postorbital region of the skull to the coronoid process of the mandible. Beneath the zygomatic branch of the masseter the pterygoideus externus (Pt. ext.) is seen. Fig. 3.—Ornithorhynchus. Mandibie seen from below, showing the attachments of the jaw muscles. Fic. 4.—The same seen from above. Fies. 5-7.—Echidna. (Muscles mainly after Schulman.) In correlation with the edentulous condition and great reduction of the jaws the temporalis (7em.) and masseter (J/us.) muscles are rather feeble. The arrangement of the muscles, however, is fundamentally the same as in Orni- thorhynchus. The posterior belly of the digastric of mammals is probably represented by the anterior part of the stylohyoideus (S/.), innervated by VII, while the anterior. belly is probably represented by the detrahens mandibule anterior (D.m.a.), which is innervated by V,, and is related to the mylohyoid (Parsons). gp ; Pane, Malia: nitigite eet ah oa éo 9 is S —e x Pa. z volte sti poet Jor aiotyntstty eee asintd eevee a f ns >? ~ he. stant vad avin 9 volves . “polo Iino caer +0 Ay ; j SS : ge ai qotoeessu oft aonb iablig toa ie a wl) Hibere f Case tah) aledibagat zugdetioh ot doidyr Yo pe ah abril sehen Sd ad botmriondi ai tnd voliiqer 10, siludibasar ‘sozeatqah sit etna. nr oh iqoesg dor ai of Wasi oT > ilsiog? out Yd Yel tour bab ors ‘ 3 ai sf s saa ajeite ny larsin aotoetl) paenee amp’: sonioinghnodi—S i out 46% 229001 hionowao: on) or ITurd oft to noior istharonea: ety wont saa is ow Sra CON ott sion as sett, ‘to dostsgsd Bila ot isonet. _oldibet ie eR . Ce ug 99 Bi (is9 A) eins i * f r , rs 7 earns ’ ay Ayn Bae “or * 28 acts * Tee eae ihe, He oo : oS .srode mort 11992 ue, ott at | ae j fi ( colds 1otte taise pafaisWE) eb, $e ort “a =H angi oli Io noitonbor 16912 bite mottibuos enolutmabo odd Mitw OL BLOTIOS | at Bogert atT .ldool toltet ore zoloznnt (awh) totezenm Dae (a9 ziferogaiot ail x @ : “ie of eg oats2 of Ylieinemeband ci r9veved, 2olseun otf) to {SURE * aldadotg ai efscensot to vitiengib oft to “fled soltetzoq ofT - Areas fou ¥ EY hotwr romtt .(.\A) eyobiordol wile ott Yo. dine toiraiae out Pee pest Lee > aati bata phe oa alt Hs aren Udadorg he vifod SOE oi otis 4 : Y ANNALS N. Y. ACAD. SCI. VOLUME XXVIII, Prats IX IXPLANATION OF PLATE X Jaw muscles of typical mammals: Didelphys, Solenodon, Mus, aud Homo In mammals the jaw muscles in front of the auditory meatus are derived from the ecapiti-mandibularis of reptiles and are innervated by the nerve V;. The depressor mandibule of the reptiles has disappeared and is functionally re- placed by the digastric which is originally a compound muscle characteristic of marsupials and placentals. The posterior belly of the digastric (Dig. p.) prebably represents a separate slip of the stylohyoideus, both muscles being innervated by closely associated branches of the seventh nerve. The anterior belly (Dig. a.) belongs with the ventral throat muscles and is innervated by the mylohyoid branch of VY. Kis. 1.—Didelphys. Superficial muscles. In correlation with the carnivorous habits and orthal jaw motion the tem poralis and masseter are both powerfully developed, while the external ptery- goid (Fig. 2) is relatively small. The: anterior and posterior bellies of the digastric (Dig. é., Dig. p.) are in the normal mammalian position. Fie. 3.—Solenodon. Superficial muscles. The masseter is considerably enlarged and its principal attachment is not on the slender zygomatic arch, but on the masseteric tubercle of the maxillary beneath the orbit. This is often the case in animals with piercing front teeth. The jaw muscles as a whole are of the carnivorous type. Fic. 4.—Solenodon. Deep muscles, showing the weak external pterygoid (Pt. ext.). Fic. 5.—Mus. Superficial muscles. The masseter is extended forward beneath the zygomatic arch in front of - the erbit, crossing the temporalis nearly at a right angle. Fic. 6.-—Mus. Deep muscles. Fic, 7.—Homo. Superficial muscles. Fic. 8—Homo. 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Ree - ; scobqnato2—b- ort se wee og eo ee 4 44 biog cro. iors anon oul saiyoule “esToxerar quale fie sean g Z Cae -_aloeitn Iniofsane one eee oe , to- Inort ai “otk oftsaron. us oid diaoned: frrn7riot habaotro ‘Bi ‘rojeedsur fo hie 2 ¢ kgs near & Is “ TROLL te acy i oift Sect’ Ge de we i oti instru one ne ut ‘ ee ; Ne y Eta : } ci) S98 7 i, oo Fae Detomtutey eitiqa ‘rode ral sbobuates aitiqns AGenertgoly Heoet 10 2 : 3 , Lael re ey iv a ~ rea . a a fa = EXPLANATION OF PLATE XII Restoration of the jaw muscles of Hryops, representing the Temnospondyli, and of Labidosaurus, representing the Cotylosauria ic. 1.—Hryops. Superficial muscles. Very probably the jaw muscles of this form were fundamentally similar to those of modern amphibians and include superficial, middle and deep layers of the capiti-mandibularis (C.m.). The pterygoideus anterior (Pt. aut.) very probably had the normal course and crossed the capiti-mandibularis on the inner side. The jaw was doubtless depressed by the depressor mandibulee (D.m.) lying behind the quadrate. Frias, 2-4.—Labidosaurus. As in the Temnospondyli (Fig. 1) the jaw muscles were entirely beneath the dermal skull roof. 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