210 THE ANATOilY OF YERTEBRATED ANIMALS. 



tinguished by the chevron-bones which are attached beneath 

 its vertebrse. The vertebrae of Ichthyosauria in general have 

 certain characters by which the}^ differ from those of all other 

 Vertebrata. (Fig. 76, C.) Not only are the centra flattened 

 disks, very much broader and higher than they are long, and 

 deeply biconcave (circumstances in w^hich they resemble the 

 vertebrge of some Labyrinthodonts and Fishes), but the only 

 transverse processes they possess are tubercules, developed 

 from the sides of these centra; and the neural arches are 

 connected with two flat surfaces, one on each side of the 

 middle line of the upper surface of the vertebrge, 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 ma}" call " neck " the most 

 anterior part of the vertebral column, the front j^art of the 

 lateral surface of each vertebra presents two separate eleva- 

 tions, or articular surfaces, which are at first situated in the 

 upper half of the lateral surface. Toward the posterior half 

 of the dorsal region they descend, and, gradually approaching 

 one another, coalesce into one in the caudal vertebrae. The 

 form of the proximal ends of the ribs corresponds with the 

 arrangement of these tubercles ; for, where they are separate, 

 the proximal end of the rib is forked. The lower fork, or 

 capitulum, goes to the capitular, or lower, tubercle, and the 

 upper branch, or tuberculum, to the upper, or tubercular, 

 elevation. In the caudal region, where the articular surface 

 is single, the proximal end of the rib is also undivided. In 

 the caudal region the ribs are short and straight, but in the 

 precaudal region they are stout and curved, and much longer 

 in the middle than at either end of the series. The atlas and 

 axis resemble the other vertebrae in their general form: but a 

 wedge-shaped bone is, as it were, let in between their opposed 

 lower edges ; and a similar bone, attached to the under-part 

 of the concave face of the centrum of the atlas, serves to com- 

 plete the cup for the occipital condyle. 



The skull of Ichthyosaurus (Fig. 7G, A) is remarkable for 

 the great elongation and tapering form of the snout, the huge 

 orbits, the great supra-temporal fossse, and the closing over of 

 the infra-temporal fossa3 by plates of bone. Again, the two 

 rami of the mandible are united in a symphysis, w^hich, for 

 length, is comparable to that observed in the modern Gavials 

 and in the ancient Teleosauria. The basi-occipital bone fur- 



