220 Prof. Owen on the Atlas, Axis, and Subvertebral 



The atlas and axis in the Fig. 3. 



Plesiosaurus (fig. 3) preserve A B 



the general proportions of J^*\S%\ rs*"**! 



the other cervical vertebra?, if|;i',, Jg|p ^& fiffl^r^wk 



and are consequently longer t^^S* cairn Ww^-^'^K 



than their homologues in jK^^B * -Iw IvvfP .J 



the Ichthyosaurus ; but they W ^^^^ ^'fr^^m ^l|fl*«*^ 



are similarly anchylosed to- ^^Bpjjjjj^w ^|fc|i^r 

 aether, and measure 4i cen- ^^SH^lES^ ^Ef 



timeters (nearly 2 inches) _ • _ > , , . _, . 



i i/L o x* i. Anchylosed atlas and axis, Plesiosaurus 



in length, 3 centimeters > ^^j,, reduced, 



across the anterior concave 



surface of the atlas, and 3^ centimeters across the less con- 

 cave posterior surface of the axis : the neural arch of each 

 vertebra has coalesced with its centrum ; and a long obtuse pro- 

 cess is formed below by a similar coalescence of the first and 

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

 centrums. The limits of the anterior ' wedge-bone/ c a, 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, c a, e x ; the anchylosed 

 bases of the neur apophyses (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 

 inferior 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, 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 atlas and axis in the Siluroid fish (fig. 1, ca ex, ex ex, &c). 

 There is no transverse process from the centrum of the atlas of 

 the Plesiosaurus ; but the fractured base of a depressed parapo- 

 physis, p (lower transverse process), or anchylosed rib, projects 

 from each side of the proper centrum of the axis. 



In a large Iguanoid lizard (Amblyrhynchus) the part answering 

 to that described by Cuvier as the body of the atlas in the Mo- 

 nitor and Crocodile* has the form of a wedge (fig. 4, ca, ex) 



* Ossem. Foss. v. pt. 2. p. 96. 



