1858.] HUXLEY — PLESIOSAURUS. 289 



the neural arch of each vertebra has coalesced with its centrum, and 

 a long obtuse process is formed below by a similar coalescence of the 

 first and second wedge-bones with each other and their respective 

 centrums. The limits of the anterior wedge-bone, ca, ex, are traceable : 

 it is proportionally larger than in the Ichthyosaurus (fig. 2), in which 

 it is likewise larger than the succeeding wedge-bones. It forms in 

 the Plesiosaurus the lower third part of the atlantal cup for the 

 occipital condyle B ca, ex: the anchylosed bases of the neurapophyses 

 (n a) form the upper border of the cup, and the intermediate part or 

 bottom of the cavity is formed by the centrum of the atlas (c a), or 

 rather by that part which, like the biconcave centrum in the Siluroid 

 fish, is developed from the central portion of the notochord. 



" The smaller or second wedge-bone (c x, e x) is lodged in the infe- 

 rior interspace between the atlas and axis, but has coalesced with both 

 bones, as well as with the large anterior wedge-bone or cortical part 

 of the body of the atlas, ca, 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 ossi- 

 fication 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. 1, c a ex, ex e x, &c). There is no transverse process from the 

 centrum of the atlas of the Plesiosaurus; but the fractured base of a 

 depressed parapophysis, p (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 irreconcileable with Professor 

 Owen's views. 



What I have observed in Plesiosaurus Etheridgii and in Plesio- 

 saurus Hawkinsii leads me, in fact, to form a very different concep- 

 tion 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. pachyomus ; 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." 



