212 



THE GEOLOGIST. 



PtychognatTius declivis, Ow.~Plane of occiput - meeting the upper (fronto- 

 parietal) plane at an acute angle, rising from below upward and backward, as in 

 the feline mammals; fronto-parietal plane bounded by an anterior ridge, ex- 

 tending fi'om one superorbital process to the other ; from this ridge the facial part 

 of the skull slopes downward in a straight line, slightly diverging from the parallel 

 of the occipital plane ; superoccipital ridge much produced and notched in the 

 middle ; the occipital plane, owing to the outward expansion of the mastoid 

 plates, is the broadest part of the skull, which quickly contracts forward to the 

 ridged beginnings of the aiveoH of the canine tusks ; orbits oblong, reniform, 

 suggestive of the reptile having the power of tm-ning the eyel)all, so as to look 

 upward and backward as well as outward. Remains of sclerotic plates. Nostrils 

 divided by a broad, flat, upward production of premaxillary, situated nearer the 

 orbit than the muzzle, smaller than in type Dicijuodon ; temporal fossae broader 

 than long, and with the outer border longest ; palate with single large oval vacuity, 

 bounded liy palato-pterygoid ridges ; occipital hypapophyses proportionally thicker 

 than in Dicynodon tigriceps ; no trace of median suture in parietal, which is per- 

 forated by a foramen parietale : fi'ontals divided by a median suture ; support a 

 transverse pan- of small tuberosities ; anterior boundaiy-ridge of vertex formed by 

 the nasals and prefrontals, the outer surface of both being divided into a horizontal 

 and slopmg facet ; lacrymal bone extending from fore-part of orbit half an inch 

 upon the face to the nostril ; premaxillary long and single, its median facial tract 

 flat, with a low median longitudinal ridge ; maxillaries forming the lower boundary 

 of the nostrils, and uniting above with the prefrontal lacrymal and nasal bones, their 

 outer surface divided by the strong ridge suggesting the subgeneric name ; teeth 

 of the upper jaw restricted to the two canine tusks, the sockets of which descend 

 much below the edentulous alveolar border ; lower jaw edentulous, deep, and 

 broad, with the fore-part of the symphysis produced and bent up to meet the 

 seemingly truncate end of the premaxillary, a character indicating, with the 

 angular outhne of the skull, the subgeneric distinction. 



Ptychognathus verticalis. — The skull of this species, repeating the subgeneric 

 characteristics of the foregoing, has the facial contour descending almost vertically 

 from, and at almost a right angle with, the fronto-parietal plane. Orbits propor- 

 tionally larger and more fully oval. Ridged sockets of the canine tusks descending 

 more vertically from below the orbits. Originals transmitted by Governor Sir 

 Geo. Gray, K.C.B., fi'om Rhenosterberg, South Africa. 



Subgenus Oudenodon, Bain (ouSeis, none, odovs, tooth). The skull in this sub- 

 genus presents the divided nostrils, the stractiure and the rounded contours of 

 that of the true Dicijnodons; also the same form, relative size and position of the 

 orbits and nostrils : but the zygomatic arches are more slender, straight and long ; 

 and, although there be an indication of an alveolar process of the superior maxillary, 

 the lower part of which projects slightly beyond the rest of the edentulous border 

 of the jaw, it does not contain any trace of a tooth, so that both jaws are edentu- 

 lous, a character which had attracted the attention of their discoverer, Mr. Bain, 

 who, in indicating it, proposed the name Oudenodon. 



It is permissible to speculate on the possibility of these toothless Dicynodontoids 

 being, after the analogy of the Narwhals, the females ; or of their being in- 

 dividuals which had lost then tusks without power of replacmg them, as 

 the known structure of the true Dicynodons indicates. But there are 

 characters of the zygomatic arches and temporal fossae which difi'erentiate the 

 toothless skull sufliciently to justify their provisional reference to a distinct 

 subgenus. 



Ilyoid apparatus of Oudenodon.— BenesAh one of the skulls, and imbedded in 

 the matrix betAveen the mandibular rami, were the following elements of the 

 hyoid apparatus :— basi-hyal, cerato-hyals, thyi'o-hyals (or hypo-branchials), cerato- 

 branchials, and uro-hyal. 



The cerato-hyals ai-e long, subcompressed, expanded at both ends ; the thyro- 

 hyals shorter and more slender ; the cerato-branchials with a sigmoid flexm-e fthe 

 uro-hyal symmetrical, broad, flat, semicircular, with a production like a stem from 

 the middle of the straight anterior margin. Tliis apparatus shows the complexity 

 by which that in Lizards and Chelonians differs fi-om the hyoid in Crocodiles, and 



