APPENDICES 385

Mytilua (Pharomytilus) plicatus (J. Sowerby),
Hdigmus cf. asiaticu$ Douville,
Mactromya cf. globosa Agassiz,
Ceratomya excentrica (Roemer),
Ceromyopsis cf. helvetica de Loriol,
Homomya inornata (J. de C. Sowerby),
PTioladomya aubryi Douville.

The above fossils include the three species found in the Nerinea
bed of Sha'ib Markh ; hence their horizon appears to be the same.
Several of the species in the list are also found in the Callovian of
Somaliland ; it is therefore probable that these beds, like those
at Haisiya, are of Callovian age.

To sum up, it may be seen that there is strong evidence for
assigning a Callovian rather than a Sequanian-Bammeridgian
age to all the fossils yet collected from the Tuwaiq Jurassic. The
well-characterised species Lopha philbyi Newton (unfortunately
a synonym of L. costellata (Douville)), originally collected by Mr.
Philby at Hamar, has not been found at other Tuwaiq localities,
but is now known to be abundant in the Callovian (and perhaps
also the Bathonian) of Somaliland, while in Sinai it marks a
horizon considered by Douville to be Upper Bathonian. The
Tuwaiq Nerinea, which is specifically distinct from—although
closely related to—N. desvoidyi, a Corallian species, was also
collected by Major H. S. Hazelgrove from near Naubat, in the
Aden hinterland.1 The age of the beds at Naubat is uncertain,
although Callovian fossils have been reported from (?) Gol Rakab,
in the hinterland of Shaqra, north-east of Aden.2 Fossils from
Dhala, some 50 miles north of Naubat, described by Newton and
Crick (loc. cit.)3 are of Lower Kimmeridgian age. This fauna of
higher horizon is characterised by Parattelodon egertonianus
(Stoliczka), Nucula wmeiformis J. de C. Sowerby, and Trochus
arabiensis Newton, which have not yet been found in the Tuwaiq
district.

(b) The Upper Cretaceous Eocks of the 'Arma Pfateau*

To the north and north-east of Riyadh, and some 30-40 miles
east of the Tuwaiq escarpment, a parallel escarpment marks the
western edge of the 'Arma plateau. At Khafs, about 60 miles
north of Riyadh, this escarpment is about 200 feet in height, but
it sinks in a southerly direction until it reaches the level of the
gravelly plain of the western part of the Rub' al Khali desert.

1 See Newton, Ann. Mag. Nat* Hist., 8th series, vol. ii., 1908, p. 9,

2 See Stefanini, Appendix to O. H. Little, The Geography and Geology of
Makalla (South Arabia), 1925, p. 194,