FOSSIL REPTILES. 361 



The forms of the vertebrae, hke those of the head, announce 

 different species. 



In individuals of the middle or small size, the length of the 

 body from front to back, in the vertebrae of the trunk, is nearly 

 one-half of its transverse diameter, but there are many much 

 thinner. 



Mr. Conybeare has represented some, the length of which 

 does not make one-third, and scarcely a fourth, of the trans- 

 verse diameter. Their absolute size also differs much. One, 

 in the Baron's possession, was five inches and a-half in trans- 

 verse diameter. Comparing them with those of an individual 

 four feet long, they indicate one of at least six-and-twenty. 



The ribs of the ichthyosaurus are very slender, for so large 

 an animal, not compressed, but rather triangular. Almost all 

 of them are bifurcated in the top, and attached to their verte- 

 brae by a head and a tuberosity, which is rather a second pedicle 

 or stem, than a second head. These existed, as in the lizards, 

 without exception, to all the vertebrae, from the head to the 

 pelvis, for the costal tubercles to the vertebrae are visible the 

 whole length of the trunk. 



It is possible that the cervical and lumbar ribs were short, 

 but those of the greatest part of the trunk were large enough 

 to take in nearly its semicircumference. Their mode of union 

 underneath, whether to the sternum, to each other, or their 

 correspondents, has not been ascertained. 



The shoulder and sternum of the ichthyosaurus are arranged, 

 in all essential points, like those of the lizards. The osseous 

 sternum is composed of an odd stem, which in front has a 

 lateral cross-piece, like a large T, and which, consequently, 

 essentially resembles its analogue in the monitor and the orni- 

 thorhyncus. To the branches of this T are attached, by a 

 suture, two clavicles, arched and tolerably strong. 



Behind this T, and partly above its odd stem, is the line of 

 meeting of the two great coracoids, cut a little fan-like, very 



