ON A NEW SPECIES OF PLESIOSAURUS 



of the body of the atlas, c a, ex. This anterior wedge-bone developes 

 a thick but short, rough tuberosity from its under part, but there is nO' 

 distinct second tuberosity from the second wedge-bone ; both indeed 

 have so coalesced together, as to parallel the continuous ossification of 

 the under part of the notochordal capsule beneath the central parts 

 of the bodies of the axis and atlas in the Siluroid fish (fig. i, caex, 

 ex ex, &c.). There is no transverse process from the centrum of the 

 atlas of the Plesiosaurus ; but the fractured base of a depressed para- 

 pophysis,/ (lower transverse process), or anchylosed rib, projects from 

 each side of the proper centrum of the axis." — (Professor Owen on 

 the Atlas, Axis, and Subvertebral Wedge-bones in the Plesiosaurus. — 

 Annals Nat. Hist. vol. xx. p. 219, 1847.) 



This is all the evidence of the nature of the atlas and axis in Ple- 

 siosaurus which is given in the paper quoted, its author seeming not to 

 be aware that important materials for checking his conclusions were 

 offered by the specimens of Plesiosaurus Hawkinsii which he had 

 already described. This is the more to be regretted, as the structure 

 of these specimens is to my mind quite irreconcilable with Professor 

 Owen's views. 



What I have observed in Plesiosaurus Etheridgii and in Plesio- 

 saurus Hazvkinsii leads me, in fact, to form a very different conception 

 of the structure of the atlas from that just cited. 



Viewed in front, the deep hemispherical articular cup of the atlas- 

 of Plesiosaurus Etheridgii is seen to be divided by a triradiate mark 

 (formed by the limestone of the matrix) into three portions ; of these,, 

 one is inferior, the other two lateral and superior. The inferior piece 

 I take to correspond with the so-called anterior or first wedge-bone of 

 P pachyo7ims ; but it forms a more considerable portion of the articular 

 cup than in the latter case, if I may judge by the figure. Viewed 

 anteriorly, this inferior piece has a semicircular contour, while seen 

 from below its anterior edge is straight, and the posterior produced 

 laterally into a sort of cornu which overlaps the sides of a second so- 

 called " subvertebral bone.'' The posterior margin is much excavated 

 in the middle, receiving the convex anterior contour of this second 

 " subvertebral bone." 



The supero-lateral pieces are separated by an interval in the 

 median line, wider than the close suture between them and the first 

 wedge-bone. At the bottom of this interval a small portion of bone 

 appears, which I believe to belong to the os odontoideum. 



After contributing their share towards the articular cup for the 

 occipital condyle, the supero-lateral pieces bend backwards so as to 

 overlap the anterior zygapophyses of the axis. They seem to ter- 



