1858.] HUXLEY — PLESIOSAURUS. 291 



the anterior concavity is well seen. The suture between the cor- 

 tical and neurapophysial portions is not traceable ; the latter slopes 

 back as in P. Etheridgii, 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. Etheridgii, 

 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 

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

 more probably with the os pdontoideum ; 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 subver- 

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

 from that ascribed to the corresponding piece in P. pachyomus. 



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. pachyomus the sutures meet below the 

 centre. 



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

 dian 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 

 neurapophyses have coalesced. In P. Etheridgii this mass is cer- 

 tainly 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 ; 

 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 be- 

 tween 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 



* Ratlike, 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 the 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." 



