FIELDIANA • ZOOLOGY Published by CHICAGO NATURAL HISTORY MUSEUM Volume 31 August 30, 1946 No. 6 A NEW OCTODONT RODENT FROM THE PARAGUAYAN CHACO Wilfred H. Osgood CuBATOR Emeritus, Department op Zoology In a small collection of mammals from western Paraguay recently presented by Mr. Boardman Conover, there is a single specimen of a tucotuco differing widely from any previously described species. It seems almost to merit generic distinction, but its affinity with Ctenomys is evident. By giving it a subgeneric position, both its distinctness and its closest alliance are indicated. It may be called Ctenomys (Chacomys) conoveri subgen. et sp. nov. Type from Colonia Fernheim, 16 km. west of Filadelphia, Para- guayan Chaco. Approximately Long. 60° 10' W., Lat. 22° 15' S. No. 54356 Chicago Natural History Museum. Adult female col- lected July 8, 1945, by Pedro Willim. Characters. — Size very large, greatly exceeding all other living species; pelage long and rather coarse; color nearly uniform through- out, including tail and feet; hind feet very broad and heavy, the claws deeply concave below and rounded at the tips; toes heavily bordered with bristly "combs." Skull massive and angular; jugal with a unique crescentic excavation in front of its high broad upward process; upper incisors very broad and heavy, slightly proodont, their anterior surfaces each with an inner and outer wide lateral groove and three shallow and narrow median grooves, the last little more than striations and not always fully continuous; cheekteeth not peculiar. Color. — General color throughout Cinnamon Rufous of Ridgway, the upper parts and sides with a light mixture of dusky and scattered white hairs, the under parts clear; heavily haired tail like back on upper side and like belly on under with a median line of white toward tip. Skull. — General appearance heavy and angular; as compared with C. robustus, one of the larger known species, there are many No. 580 47 48 FIELDIANA: ZOOLOGY, VOLUME 31 differences. The rostrum is much broader and heavier, the nasals nearly twice as wide, and the zygomata much heavier; the jugal is large, with a high broad ascending process the anterior part of which has a deep excavation carried forward in crescentic or almost semi- circular form to the maxillary; the frontals and parietals are fully ankylosed medially and a definite groove runs from the posterior part of the frontal to the occiput; ridges from the postorbital proc- esses to the occiput form a parietal shield with protuberances about midway of their length instead of the usual slightly converging ridges found in other known species. The audital bullae are quite oblique, long and narrow rather than short and bulbous as, for example, in C. rohustus. The cheekteeth are of good size and have the pattern usual in Ctenomys. Fig. 4. Ctenomys (Chacomys) conoveri, type. X 1. Measurements. — Adult female type measured by collector: total length 425; tail 105; hind foot 60. Skull of type: greatest length 69.7; basilar length 56.2; occipito-nasal length 58.6; zygomatic breadth 48.4; nasals 24.4 x 13.9; interorbital breadth 17.6; breadth across post- orbital processes 21.5; postglenoid breadth 23.9; breadth across audital bullae 38.6; length and breadth of audital bullae 24, 9; diastema 18.6; cheekteeth 15; length of anterior cheektooth 6.1; width of upper incisor 5.5; combined width of upper incisors at base 13.9. Remarks. — The single example of this animal is much larger than any Ctenomys previously described, but, since it is a female, doubtless it falls considerably short of the maximum size of the species. Several extinct forms have been named, mostly of relatively small size with the possible exception of Megactenomys (Rusconi, Anales Soc. Sci. Argentina, 110, pp. 251, 254, 1930) based on the anterior part of a skull including rostrum, incisors, and one premolar. The length of the diastema is given as 18 mm., almost equalling that in the form OSGOOD: A NEW OCTODONT RODENT 49 here described, but the rostrum is narrow, the incisors smooth-faced, and the premolar very small ; so any close relationship seems unlikely. Other extinct forms include Dicoelophorus Ameghino (or Acte- nomys), Eucelophorus Ameghino, Paractenomys Ameghino and Xenodontomys Kraglievich. Some of these are well characterized, while others as described seem scarcely distinguishable from the recent Ctenomys and none of them offers any especial resemblance to Chacomys. The usual number of mammae in Ctenomys is six, two pairs abdominal, and one pair inguinal. In the type specimen here described these are quite evident, but in addition there is a single one in subpectoral position rather high on the left side but no cor- responding one on the right side. This seems to indicate that the mammae normally may be six or eight. The grooved upper incisors in this species are quite unique since no suggestion of them is found in any species of typical Ctenomys. Comparison with any of the larger known species reveals innumer- able cranial characters, some trivial and others perhaps of con- siderable importance. A somewhat pronounced character appears in the deep semicircular excavation in the upper part of the jugal. In most species of Ctenomys this part presents a plane surface or even a slightly convex one. In a few others, notably C. steinhachi, traces of a similar depression occur. In the present species the structure is so pronounced as to suggest the attachment of a peculiar muscle, the function of which is only conjectural. Apologies are perhaps in order for the "hybrid" name Chacomys. It may be said, therefore, that brevity, euphony, and significance seem to give it preference over such five-syllabled combinations as Paractenomys and Megactenomys, neither of which is available.