Mr. L. Barrett on the Atlas and Axis of the Plesiosauras. 363 



the bodies of the two vertebra? have so coalesced, that the original 

 line of separation is scarcely visible. The neurapophyses and 

 cervical ribs of the axis have become anchylosed to the bodies of 

 that vertebra. The posterior half of the bottom of the neural 

 canal in the atlas is formed by the true centrum of that vertebra ; 

 but in the anterior half the bases of the neurapophyses have 

 spread over the centrum, and have united at the medial line. 

 On the upper part of the atlantal cup a groove indicates the 

 position of the original suture between the bases of the neura- 

 pophyses and the lower part of the atlantal cup. In the larger 

 specimen there is a trace of the suture which separated the 

 anterior subvertebral wedge-bone from the upper part of the 

 atlas, but which is absent in the smaller specimen. 



In the atlas of the three species of Plesiosaurus we have now 

 considered, the anterior articular face of the atlas is made up of 

 four bones : of these, the os odontoideum is the most variable in 

 size. Prof. Owen correctly assigned to it a large share in the 

 formation of the atlantal cup in the Kimmeridge Clay species. 

 It forms about a third of the upper half of the cup in the young 

 unanchylosed specimen, and in P. Etheridgii is extremely small. 

 Its position varies : in the young specimen (figs. 1 & 1 a) it 

 forms the base of the neural canal of the atlas ; in the Kimmeridge 

 Clay species (figs. 3 a & 3 b) it is overlapped by the expanded 

 bases of the neurapophyses ; and in P. Etheridgii it occupies the 

 centre of the cup. The bases of the neurapophyses of the atlas 

 are most developed in this species, and least in the Kimmeridge 

 Clay species ; in all cases the anterior subvertebral wedge-bone 

 forms a large portion of the atlantal cup. That the suture be- 

 tween this bone and the os odontoideum, in the atlas of the 

 species figured by Prof. Owen in the { Annals ' for 1847, is cor- 

 rect, we have abundant proof in the structure of the atlas of 

 Pliosaurus, where the os odontoideum is of exactly the same 

 shape, and the wedge-bone separated from it by a similar suture. 



It is remarkable that the Kimmeridge Clay species approaches 

 more nearly the Ichthyosaurian type than the Lias species, not 

 only in the greater development of the os odontoideum and in 

 its lateral edges forming the lateral margins of the atlas, but in 

 its supporting the neurapophyses ; there is really no essential 

 difference between the atlas of this species and the atla3 of 

 Ichthyosaurus. 



The atlas of P. Etheridgii and that figured (figs. 1 & 1 a) 

 show many Crocodilian affinities (the neurapophyses being sup- 

 ported both by the wedge-bone and the centrum) ; but the pos- 

 terior edge of the centrum does not support a pair of ribs 

 (pleur apophyses), and no trace of ribs articulating with the 

 wedge-bone have been discovered. 



