706 PBOF. OWEN ON THE RANK AND AFFINITIES IN 



This character materially aided in the determination of the Ophi- 

 dian nature of the Eocene vertebrae described in the ' Transactions 

 of the Geological Society,' vol. vi. 2nd Series, 1839, p. 209, pi. xxii. ; 

 and the character in question is again illustrated in the ' Proceedings ' 

 of the Society for January 7th, 1857, p. 196, pi. iv., in the extinct 

 genera Palceoplris and Laophis, and in the existing genera Python, 

 Coluber, and Crotalus. By virtue of these superadded processes 

 (zygosphene and zygantrum) the vertebras interlock by a double 

 " tenon-and-mortice w joint. 



In the course of the comparisons for determining the fossil ver- 

 tebrae submitted to me from the Eocene clay of Sheppey and Brackles- 

 ham *, and from an undetermined Tertiary formation at Salonica, 

 I came upon a similar complexity in the arboreal or terrestrial 

 lizards of the genera Iguana and Metopoceros ; and to this day they 

 form, as far as my examinations have extended, the sole exceptions 

 in the Lacertian order. It was with much interest therefore that 

 I availed myself of the opportunity of examining the vertebral 

 characters of the great marine Iguanian lizard of the Galapagos 

 Islands, in the specimen from Iguana Cove, Charles's Island, 

 presented to the British Museum by Commander Cookson, R.N. 

 The dimensions of the skeleton are given above, Its vertebras difTer 

 from those of Iguana proper, and retain the simplicity of articulations 

 common, with the exceptions noted, to all Lizards. I know of no 

 exceptions to the rule of the complexity of the vertebral articulations 

 in Ophidia, unless Mosasaurus be shown by this character to be a 

 a great sea-serpent. 



But then it might be objected, if vertebrae of any mosasaurian species 

 should show the zygosphene and zygantrum or their rudiments, that 

 these would exemplify, like the pterygoid teeth, the affinity of such 

 exceptional species to the Iguanian Lizards. 



In Amblyrhynchus and most other Lacertians the neurapophyses 

 of the atlas touch each other above by a small surface, and then, 

 commonly, coalesce. In Mosasaurus they appear to retain their 

 individuality, at least in the fossils. The neurapophyses narrowing 

 to a blunt point above have been found ununited there, and usually 

 pushed or fallen asunder, as in the specimen of Mosasaurus Hoff- 

 manni figured by Hoffmann and Cuvier (torn. cit. pi. xx. fig. 14, a a). 

 Cuvier conjectured that the upper interspace might have been closed 

 by a distinct piece (flattened neural spine), as in Crocodilia ; but no 

 part answering to this was in the Breda collection, nor has been 

 recognized in the American specimens. In the latter, as in the 

 Maestricht fossils, the atlantal neurapophyses are ununited above f. 



Python differs from Amblyrhynchus and Bydrosaurus in the much 



* Monograph on the Crocodilia and Opkidia of the London Clay, p. 53, pi. 

 xiii. figs. 33, 36, vol. of Paleontographical Society for 1850. 



t Prof. Cope writes : — " The atlas consists of a basal and two lateral pieces 

 only" {op. cit. p. 114). And again (p. 122) : — " The atlas consists of the three 

 pieces, the basal and two lateral." He does not notice the differential character 

 in Python, nor recognize, as Camper rightly did, the odontoid process as the 

 body of the atlas (comp. Cuvier, loc. cit. p. 330). 



