362 Mr. L. Barrett on the Atlas and Axis of the Plesiosaurus. 



That part of the centrum which forms the middle of the upper 

 half of the atlantal cup (fig. 1, c.) is hexagonal, and it has a small 

 pit in its centre ; posteriorly its articular face is three times as 

 great (nearly as large as the articular surface of the body of the 

 axis), and has a circular depression in the middle. The wedge - 

 bone (fig. 1) forms the lower half of the articular cup, and has 

 been produced behind into two long processes, the bases of which 

 only remain. 



The neural spine of the axis (fig. 2, n. s.) is long, and much 

 thicker than that of any of the succeeding cervical vertebrae ; 

 the apex is broken off in this specimen. The neurapophyses are 

 separated from the centrum by a distinct suture ; and an oblique 

 ridge connects on each side the anterior with the posterior zyg- 

 apophysis. The antero-inferior edge of the axis is bevelled-off, 

 forming an articular surface for the second wedge-bone ; and the 

 basal portions of two cervical ribs (figs. 2 & 2 a, p. l.) remain 

 attached to the anterior lower part of the centrum : they must 

 have partly articulated with the second wedge-bone. 



The axis of Plesiosaurus Etheridgii, lately examined by Prof. 

 Huxley, agrees entirely with that of this species ; but there are 

 some important modifications in the structure of the atlas. 



Prof. Huxley describes the atlantal cup in this species as being 

 divided by a triradiate mark into three portions — one inferior 

 and two lateral and superior. The inferior piece corresponds 

 with the lower half of the atlantal cup, or the anterior subver- 

 tebral wedge-bone, and the two supero-lateral pieces to the neur- 

 apophyses in the specimen first described, but their bases are 

 much more developed. 



There is a small circular bone in the centre, which Prof. Hux- 

 ley considers to belong to the os odontoideum ; it is the anterior 

 articular face of that bone, and corresponds to the hexagonal 

 bone in the middle of the upper half of the atlantal cup in the 

 former species, — the difference in position being caused in this 

 species by the great development of the supero-lateral pieces or 

 bases of the neurapophyses. 



The postero-lateral edges of this bone are greatly developed, 

 forming a rounded ridge on both sides of the posterior part of 

 the atlas : the extraordinary development of this part of the 

 bone is the most remarkable feature in the atlas and axis of this 

 species. 



We now come to the species first described by Prof. Owen. 

 We have two specimens, in the Woodwardian Museum, of this 

 species, both from the Kimmeridge Clay of Haddenham, near 

 Ely ; the larger of the two was figured by Prof. Owen in the 

 ' Annals of Natural History/ vol. xx. 



The neural arches in both specimens are broken away ; and 



