304 



adult animal, as in PL priscus : yet the general 

 analogy of the bones, as far as hitherto ascertained, 

 seems to warrant their being regarded as a distinct 

 species. It has not yet been described. 



annular suture ; at the same time, the stems by which the annular 

 part was attached to the body expand their bases laterally, so as to 

 form incipient transverse processes. All the vertebrae, from the first 

 developement of the transverse processes (twenty-seven in number), 

 carry the ribs on a single articulating surface at the end of that 

 process, like the three last dorsals in the crocodile, and the whole 

 series in most other saurians. The pointing upwards of the trans- 

 verse processes in the middle of the series seems intended to give a 

 wider sweep to the ribs flanking the thorax and the abdomen. 



" The lumbar and caudal vertebrae appear to differ in form from 

 one another only in a less inflection of the lower margin, for the 

 purpose shortly to be mentioned ; they are, however, very distinct 

 from all the others ; they have no regular transverse processes, but, 

 instead of them, two separate bones flattened at the extremities, 

 and articulated into a socket near the upper part of the sides of the 

 bodies of the vertebrae ; their position is exactly horizontal. The 

 ichthyosaurus appears to have had similar bones. Tlie lower margin 

 of these vertebrae on the posterior extremity is inflected so as to 

 form two regular indentations, exactly as in the crocodile, for the 

 reception of the chevron-shaped bone beneath the tail. The middle 

 dorsals are considerably larger than those of the extremities of the 

 column. There have been, as yet, no means of even surmising the 

 number of caudal vertebrae. " 



It appears, then, that the vertebral column of the plesiosaurus 

 recedes from that of the ichthyosaurus in all the points in which the 

 latter approaches to the fishy structure ; that the intervertebral sub- 

 stance must have been disposed much as in the cetacea, and that on 

 this account, as well as because the annular parts were firmly at- 

 tached to the bodies, and, therefore, by the locking into one another 

 of their articulating processes, must have given a considerable degree 

 of stability to the column, it must have possessed, in a much less 

 perfect manner, the flexibility which facilitates the peculiar motion 

 of the ichthyosaurus and of fishes. But this was much less neces- 

 sary to these animals, inasmuch as the structure of their extremities 

 rendered them much more powerful instruments of progression. 



