Wedge Bones in the Plesiosaurus. 219 



the above- described ossification of the anterior end of the central 

 part of the notochord. If, on the other hand, such ossified part 

 of the notochord were to coalesce with the basi-occipital instead 

 of with the atlas, it would form a tubercle on the back part of 

 the occipital centrum which would fit into the concavity left on 

 the fore-part of the centrum of the atlas. 

 Now this is precisely what has happened 

 in those large extinct fish-like reptiles, the 

 Enaliosauria (fig. 2). That is to say, the 

 basi-occipital presents a convex condyle 

 (c 6) which is received into a cavity on the m*^SSa3 

 fore-part of the body of the atlas ca, com- Anterior cervical verte- 

 pleted below by the first ' wedge-bone ' c a, brae, Ichthyosaurus, 

 ex. The main or central part of the body of the atlas, ca, 

 as Sir P. Egerton has shown, is early anchylosed to the body 

 of the axis, c x ; and, in a specimen in which he succeeded in 

 separating the two vertebrae, they were applied to one another 

 by flat and even surfaces. Into the lower part of this speedily 

 obliterated symphysis a second distinct ossicle (c x, e x) is wedged, 

 a similar but smaller ossicle (c 3, e x) being situated at the infe- 

 rior interspace between the axis and third vertebra. 



The condition of the anterior vertebrae of the large Siluroid 

 fish (fig. 1), in which I found the central biconcave parts of the 

 bodies of the atlas, axis, and three succeeding vertebrae established 

 by distinct ossification of the central part of the notochord, whilst 

 the whole were attached below to a continuous ossification in the 

 capsule of the notochord, will explain what is meant by the state- 

 ment that the subvertebral wedge-bones of the Ichthyosaurus are 

 derived from " detached developments of bone in the lower part 

 of the capsule of the notochord," at the inferior interspace be- 

 tween the occiput and atlas, and at the similar interspaces of the 

 two or three succeeding cervical vertebrae ; but varying in num- 

 ber in different species. 



A recent opportunity of examining the atlas and axis of the 

 Plesiosaurus, kindly afforded me by my friend Prof. Sedgwick, has 

 not only strengthened this view of the general nature of the ' sub- 

 vertebral wedge-bones/ but has made me incline to the second 

 hypothesis of the special homology of the first or anterior of the 

 wedge-bones, which is proposed in my ' Report on British Fossil 

 Reptiles/ viz. that it answered to the part described as the body 

 of the atlas in the existing Saurians and Chelonians; which there- 

 fore may be regarded, like the first subvertebral wedge-bone, as 

 the cortical part only of such vertebral body, like the plate of bone 

 beneath the biconcave central part of the body of the atlas in the 

 Siluroid fish. 



16* 



