684 PROF. OWEN ON THE RANK AND AFFINITIES IN 



(basisphenoid) in front. In Ophidia also the hypapophysis of the 

 basioccipital is single and median. In Python tigris it is " pro- 

 duced into a recurved point " *. In Crotalus horridus the single 

 basioccipital hypapophysis forms the commencement of the strong 

 ridge from the underpart of the basisphenoid. In all Lacertilia 

 the basioccipital has a pair of hypapophyses. In the Monitors 

 (Varanus) they are short, obtuse, and slightly divergent f. In 

 JRhynchocephalus J they are more lateral and divergent, abutting 

 against the ends of the long pterygoids. In Iguana they more 

 closely agree with the parial hypapophyses in Mosasaurus ; and the 

 basioccipital of Iguana tuberculata (Quart. Journ. Geol. Soc. vol. v. 

 pi. x. fig. 6), with the homologous bone of Alligator Indus (ib. ib. 

 fig. 7), were selected to illustrate the character of the basioccipital 

 in Mosasaurus Maocimiliani (ib. ib. fig. 5). In the great existing 

 Sea-Lizard (Amblyrhynchus cristatus, Bell), the skeleton of which I 

 had not then seen, the correspondence of the basioccipital with that 

 of Mosasaurus is closer than in any other Lacertian §. 



As the idea of an aflinity of the Mosasaurian Reptiles with 

 Serpents, whether marine or terrestrial, had not been broached in 

 1848, nor seemed likely to occur to a palaeontologist from the data 

 then at his command, no figure of the basioccipital of a Python was 

 thought requisite. 



The elements in the determination of the place of the Mosasaurians 

 in the Reptilian series, additional to those discussed in detail by 

 Cuvier ||, relate chiefly to the hind and fore ends of the skull, to its 

 upper surface, and to part of the lower surface. I proceed to 

 notice such supplementary evidences as have come under observa- 

 tion since the date of the paper quoted in the Society's Quarterly 

 Journal. 



The basisphenoid is a more characteristic bone in the present 

 question than the basioccipital. In Ophidia its under surface is 

 traversed by a single median hypapophysis in the form of a sharp and 

 deep ridge %. In Lacertilia the corresponding surface of the basi- 

 sphenoid is devoid of such ridge, is smooth, and slightly concave. 

 A pair of hypapophyses from the posterior angles abut against those 

 of the basioccipital. In Mosasaurus the basisphenoid is concave, 

 almost canaliculate along the middle of the under surface, devoid of 

 any ophidian median ridge ; the pair of hypapophyses, or produced 

 posterior angles abut against the pair from the basioccipital, but 

 leave the broad truncate ends of these free, as in Amblyrhynchus. 

 The " pterapophyses " in Lacertilia are subcompressed laterally, 



* Descriptive Catalogue of the Osteological Series in the Museum of the 

 Royal College of Surgeons,' 1853, p. 128. no. 628. 



t Anat. of Vertebrates, vol. i.p. 155. 



\ Ibid. p. 154. 



§ Comp. Steindachner, ' Die Schlangen und Eidechsen der Galapagos-Inseln,' 

 1876, pp. 313-338, pis. xviii., xix. 



J Tom. cit. 



^| Osteol. Catal. cit. Python tigris, p. 129. no. 628; Crotalus horridus, 

 p. 135. no. 640: "the strong ridge developed from the underpart of the basi- 

 *phenoid " is specially noticed. 



