712 PROF. OWEN OX THE RANK AND AFFINITIES IN 



acrodont character is developed in the dentition of the Mosasauroids 

 are a family characteristic. The root, as well as crown, is developed 

 before the sncccssional tooth is extricated or protruded. Camper 

 observed and rightly noted this fact : " A small secondary tooth is 

 formed complete with its enamel and solid root in the osseous substance 

 of the temporary tooth." 



§ 11. Skin. — In the ' American Journal of Science and Arts,' 

 April, 1872, Prof. Marsh announced his discovery of fossilized dermal 

 scutes in Mosasauroid reptiles ; their preservation in a petrified 

 state implies their osseous basis ; and their imbricate arrangement 

 and dimensions are noted. The epidermal layer which they pro- 

 bably supported would add to their thickness. This discovery adds 

 an important element in determining the affinities of Mosasaurus, 

 especially within the limits of the present inquiry. The scales in 

 certain Lacertilia have a bony basis. In all Ojrfudia they are 

 epidermal only, and unfossilizable. 



§ 12. Conclusion. — The foregoing comparisons lead to a retrospec- 

 tive glance at the period when any evidence of the extreme modifica- 

 tion of the Reptilian type to which the term " ophidian " can be 

 intelligibly given, has been recognized. 



I am not cognizant of such prior to the Tertiary epoch. There 

 are indications of anguiform Saurians in the lithographic slates ; and 

 the DoUcJwsaimis * of the Chalk shows a number of trunk-vertebrae 

 equalling those of the snake-like Lacertians (Pseudopus, Ophisawrus, 

 e. g.) ; but genuine ophidian characters are wanting. They were 

 first made known to the writer by the vertebrae of Palceojphis from 

 the London and Bracklesham clays. 



Characters which have been noted as lacertian modifications of the 

 Reptilian class begin to appear pretty early in the series : — the parial 

 hypapophyses of the basioccipital f for example; the bony connex- 

 ion of the paroccipital with the exoccipital, forming the long and 

 strong beam of bone extending from the side of the occiput to abut 

 against the mastoid and tympanic % '■> the posterior bifurcation of 

 the parietal, the prongs of which diverge and support the mastoids § ; 

 the foramen parietalo || ; the length, slenderness, and elevated posi- 

 tion of the zygoma, the fore pier of which arch is formed by both 

 maxillary and postfrontal, and the hind pier by the mastoid and 

 upper end of the tympanic % : all these cranial characters are com- 

 bined with those of the theriodont and anomodont orders in the 



* Dixon's ' Geology of Sussex,' &c, 1850, p. 389. 



t Dicynodon lacerticeps, ' Descriptive and Illustrated Catalogue of the Fossil 

 Reptilia of South Africa,' 1876, pi. xxiii. fig. 3, 1',1'j D. leoniceps, ibid. pi. xxvi. 

 fig. 2, v, V. 



{ Ptychoynafhus, ibid. pi. xlvi. figs. 1 & 2, 2, 4. 



§ Oudenodon, ibid. pi. lx. fig. 2, 7, 8. 



| Galesaurus, ibid. pi. xviii. fig. 8, 7'. Cuvier notes a Liassic instance of this 

 character in his classical chapter on the Mosasaurus :— " On doit remarquer un 

 trou qui est naturellement perce dans le parietal a peu pres \ ers le milieu, et qui 

 se retrouvera dans beaucoup d'autres sauriens, et j usque dans V Ichthyosaurus." 

 — Torn. cit. p. 257. • 



r Oudenodon, ibid. pi. lx. fig. 1, 21, 12, 2C>, 27, 8, 28. 



