ON A NEW SPECIES OF PLESIOSAURUS 535 



the so-called body of that bone (the homologue of the inferior piece 

 in the Plesiosaurus) being a distinct peripheral ossification. 



I have just shown, however, that in P. Etheridgii the os odontoi- 

 deum must be sought elsewhere, and I have not the slightest doubt 

 that it is that osseous plate whose convex lateral edges are seen 

 between the anterior portions of the atlas and the axis, and which 

 ends below in the so-called second subvertebral bone. 



6. I have not as yet met with any neural spine in the atlas of 

 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 t\\-o structures 

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

 of his " Crocodile d'Honfleur " (a Teleosaurian) " soudds ensemble," 

 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 Plesiosaurus?- 



The Structure of the Cranium. — The length to which these remarks 

 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. 



I. 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. 



^ ' Ossemens Fossiles,' ed. 4, t. ix. pp. 306-7. 



''' Hermann von Meyer says ('Die Saurier des Muschelkalkes ') of Nothosaui-iis : " The 

 atlas had a remarkably depressed superior arch, whose spinous process was inchned 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 interspace 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. 



^ My statements respecting the structure of the skull of Teleosaurus are based on my 

 examination of two very beautiful specimens of 7'. 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. 



