534 ON A NEW SPECIES OF PLESIOSAURUS 



well, but the latter is somewhat crushed and distorted ; nevertheless; 

 the anterior concavity is well seen. The suture between the cortical 

 and neurapophysial portions is not traceable ; the latter slopes back 

 as in P. Etlieridgii, and is visible on the left as well as on the right 

 side. The peripheral piece appears to be prolonged anteriorly instead 

 of posteriorly, but I believe this to arise from crushing merely. 



The edge of an interposed bony plate is seen, as in P. Etlieridgii^ 

 between the posterior edge of the three anterior portions of the atlas 

 and the body of the axis. The neural spine of the axis is long and 

 recurved ; there is a rib with a short and broad head, which is articu- 

 lated for the greater part of its extent either with the axis, or more 

 probably with the os odontoideum ; its anterior angle extends forwards 

 as far as the inferior piece of the atlas. 



Putting these different views and sections of the atlas and axis of 

 Plesiosaurus together, it seems to me that they are consistent with 

 only the following interpretation : — 



1. The atlas and axis are, as Prof Owen states, anchylosed. 



2. What I have called the inferior piece of the anterior part of the 

 atlas, corresponds with what Prof Owen terms the anterior subverte- 

 bral wedge-bone ; but I find its shape to be exceedingly different from 

 that ascribed to the corresponding piece in P pachyoinus. 



3. The sutures between this and the supero-lateral pieces are 

 situated at a higher level on the face of the articular cup in P. 

 Etheridgii. They are here, in fact, radii from the centre of that cup,, 

 while in the figure of P pachyoinus the sutures meet below the centre.. 



4. Prof Owen describes no distinct supero-lateral pieces or median 

 suture ; and, not having seen them, he considers the upper two-thirds 

 of the cup to be formed by a distinct mass, with which the neura- 

 pophyses have coalesced. In P. Etheridgii this mass is certainly 

 nothing more than the bases of the neurapophyses themselves, which 

 contribute, as in the Crocodile, to form the articular surface for the 

 occipital condyle. 



5. Prof Owen conceives that the upper two-thirds of the articular 

 cup (all but its extreme margin ?) constitute the homologue of the 

 OS odontoideum, which is (as Rathke ^ proved eighteen years ago)> 

 simply the separately ossified central portion of the body of the atlas ;, 



1 Rathke, ' Entwickelungs-geschichte der Natter,' 1839, pp. 119, 120; also ' Ueber die 

 Entwickelung der Schildkroten,' 1848. In the former essay Rathke says of the " processus 

 odontoideus," "Therefore this process is not an outgrowth of the epistropheus, but tlie body 

 of the atlas ; while that bone which is reckoned as the first cervical vertebra is not a perfect 

 vertebra, having no true body. What is called its body is nothing but a modified inferior 

 spinous process. " 



