Cervical Vertehrce of the Ichthyosaurus. 1 89 



smaller plane (fig-. 2. c.) proceeding downwards and forwards from the pos- 

 terior margin of the atlas. The third of like shape and size with the second, 

 (fig. 2. /.), extends from the anterior margin of the axis, and joins the 

 apex of the fourth (fig. 2. g.) which inclines forwards from the posterior 

 portion of the same bone. This fourth plane is considerably smaller than the 

 others, and corresponds in size with a fifth (fig. 11. a.), placed on the anterior 

 border of the third cervical vertebra. When, therefore, the three anterior 

 vertebrae are in their natural position, the arrangement of the five planes is as 

 follows. The first and largest occupies the lower front of the atlas ; the 

 second and third, by the union of their bases, produce a triangular socket on 

 the under surface of the atlas and axis ; and a second smaller socket is formed 

 between the axis and the third vertebra, by a similar disposition of the fourth 

 and fifth planes. Having endeavoured to render intelligible the arrangement 

 of these parts, I shall proceed to describe three peculiar bones which articu- 

 late with the above-mentioned planes, and which I propose, from their form 

 and position, to designate subvertebral ivedge bones. 



The fortunate acquisition of an Ichthyosaurus having the first subvertebral 

 bone in situ, enables me to omit a series of arguments by which I was pre- 

 pared to show, that a bone did exist supplementary to the atlantal socket, and 

 that the specimen represented at Plate XIV. figs. 4, 5, 6. was to be ascribed 

 to this position in the skeleton *. The second bone of the series is frequently 

 found with the atlas and axis, and is not uncommonly fixed in its position by 

 anchylosis. The third bone is of rare occurrence, in consequence of its 

 diminutive size, and is probably in some species altogether wanting. Fig. 7. 

 is an accurate representation of the cervical apparatus of the skeleton I have 

 above mentioned, and shows the three subvertebral bones so far displaced as 

 to allow their respective sockets to be seen. Fig. 8. a. b. c. is an enlarged 

 view of the same bones seen from below. 



First Subvertebral Wedge Bone. 



This bone is of great strength and solidity. Its form triangular. The under surface is thick- 

 ened into a central projecting boss, Plate XIV. figs. 5&:6. a., from which it spreads out anteriorly 

 into two alae, representing two angles of the triangle. (Figs. 4, 5, 6. b.) The base between these 

 points is occupied by a smooth semilunar socket (fig. 4. c.) which receives the lower hemisphere 

 of the articulating tubercle of the occiput. A rapid contraction of the bone posteriorly forms 

 the apex of the triangle which is slightly truncated, (figs. 5, 6. d.) The upper portion of this 



* I have just seen in the magnificent collection of Mr. Hawkins an Ichthyosaurus from the lias 

 of Somersetshire, having the first and second bones in their natural position. 



