THE ICHTHTOSATTEIA. 243 



Ichthyosaurus may have been provided with a soi't of finlike 

 expansion of the integument. This fishhke body was pro- 

 pelled, like that of the Plesiosaums, by four paddles ; but 

 the anterior paddles were placed close behind the head, and 

 were, generally, very much larger than the posterior ones. 



Fig. 75. 



Fig. 75. — A restoration of Ichtliyosaurui. The existence of the caudal 

 fin is doubtful. 



The spinal column is only distinguished into two regions, 

 caudal and precaudal, inasmuch as the ribs, beginning at the 

 anterior pai't of the neck, are continued, without being 

 connected with the sternum, to the posterior end of the 

 body ; and there is no sacrum. The caudal region, however, 

 is distinguished by the chevron bones which are attached 

 beneath its vertebrae. The vertebrae of Ichthyosauria in 

 general have certain characters by which they diifer from 

 those of all other Vertebrata. (Fig. 76, C.) Not only are 

 the centra flattened discs, very much broader and higher 

 than they are long, and deeply biconcave (circumstances 

 in which they resemble the vertebra of some Labyrintho- 

 donts and Fishes), but the only transverse processes they 

 possess are tubercles, developed from the sides of these 

 centra ; and the neural arches are connected with two flat 

 stu-faces, one on each side of the middle line of the upper 

 surface of the vertebrae, by mere synchondroses. The neural 

 arches themselves are forked bones, with only rudiments of 

 zygapophyses, and in the greater part of the body do not 

 become articulated with one another at all. 



In the cervical region, if one may call " neck " the most 

 anterior part of the vertebral column, the front part of 

 the lateral surface of each vertebra presents two separate 



