292 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 6, 



Plesiosaurus corresponding with the flattened and separate homologue 

 of this part of a vertebra which is found in the atlas of the Crocodile ; 

 but the complete correspondence of: the Plesiosaurian atlas and axis (in 

 this reading of their structure) with those of the Crocodilia is highly 

 interesting, as it harmonizes perfectly with the strongly crocodilian 

 affinities manifested by many other parts of the organization of the 

 Plesiosaurus. I reserve a lengthened comparison of the two struc- 

 tures for my Memoir, merely adding that Cuvier found the atlas and 

 axis of his "Crocodile d'Honfleur" (a Teleosaurian) "soudes en- 

 semble," the posterior face of the axis being concave* ; and that, 

 according to Von Meyer's figures, the atlas and axis of Nothosaurus 

 were very similar to those of Plesiosaurusf. 



The Structure of the Cranium. — The length to which these re- 

 marks have already extended, and the impossibility of rendering any 

 account of the structure of the cranium intelligible without a large 

 number of illustrations (which will be more fitly reserved for my 

 forthcoming memoir), lead me to throw what I have to say into a 

 few propositions, whose full proof will be adduced hereafter. 



1. The structure of the Plesiosaurian cranium is best to be under- 

 stood by comparing it with that of Teleosaurus%, when its numerous 

 crocodilian affinities become at once apparent. 



2. In the Teleosauria (Teleosaurus temporalis) there is a singular 

 aperture closely resembling in form and position the external nostril 

 of the Plesiosaurus, though in the Teleosauria there is every reason 

 to believe that the nostrils were, as in the Gavials, at the end of the 

 snout. The bony margins of the aperture are, however, somewhat 

 differently constituted in the two genera. 



3. In the Teleosaurus the jugal bone is long and slender. In 

 Plesiosaurus Etheridgii and others, I find a bony style of greater 

 or less length, broken posteriorly, but having otherwise precisely the 

 same relations and form as the jugal of the Teleosaurus §. This 

 process is particularly well shown in a cranium (named P. dolicho- 

 deirus) in the Museum of the Society, and is figured by Mr. Cony- 

 beare in his restoration. 



* Ossemens Fossiles, ed. 4. t. ix. pp. 306-7. 



f Hermann von Meyer says (Die Saurier des Muschelkalkes) of Nothosaurus : 

 " The atlas had a remarkably depressed superior arch, whose spinous process was 

 inclined backwards at an angle of about 25°. The posterior articular processes 

 were directed backwards ; and below, a short lateral part, analogous to a hook-like 



cervical rib, appears to have been attached The atlas and axis do not 



seem to have been anchylosed." (p. 30.) On comparing Von Meyer's figures, the 

 similarity of the atlas and axis to those of Plesiosaurus is remarkable. The inter- 

 space left between the axis and atlas corresponds to that for the os odontoideum, 

 and the projecting piece figured at the lower anterior edge of the axis may, I 

 think, very possibly be the free lower end of the os odontoideum. 



X My statements respecting the structure of the skull of Teleosaurus are based 

 on my examination of two very beautiful specimens of T. temporalis in the 

 Tesson Collection, now in the British Museum. I am informed that these crania 

 were worked out from the matrix by M. Selys Deslongchamps, to whom therefore 

 the credit of the discovery of any new points is properly due. 



§ Hermann von Meyer figures a very similar process in Simosaurus, pi. 65. 

 figs. 1 & 2. 



