STRUCTURE OF FISHES. 377 



In the Ray and the Shark tribes, both the ante- 

 rior and posterior extremities are supported by 

 arches of bones, forming a sort of belt. This struc- 

 /^ 193 ^. ture is an approach to 



^^^^i^/f m.^-^ ^^^ which obtains in 



"^ ^5^^^^^-^=^^^^*^^^ many reptiles, and indi- 

 ^•"-^ % cates a lurther step in 



the progress of developement. This belt in the 

 Ray is shown in Fig. 193. 



In examining that part of the skeleton of fishes 

 which corresponds to the posterior extremity, we 

 observe the total absence of both femur and tibia ; 

 but the bones of the toes are attached [to a set of 

 small bones, which appear to act the part of a 

 pelvis, but which, in consequence of their not being 

 connected with the spine, have no determinate 

 situation, and are found at various distances from 

 the head in different fishes. They appear emanci- 

 pated from the restraints to which they would have 

 been subjected had they been fixed to a sacrum, 

 or to any particular part of the spine ; we find 

 them, accordingly, often placed considerably for- 

 wards ; and in some instances, as in the Siihbra- 

 chieni, even anteriorly to the pectoral fins, which 

 are the true arms of the animal. But in one whole 

 order of fishes, the Apodes, there is not even a ves- 

 tige of ventral fins, nor are any pelvic bones pro- 

 vided for their support. This is the case with the 

 Eel, the Gymnotus, &c. In a few species there is 

 also a total absence of pectoral, as well as ventral 

 fins. 



The dorsal fins are supported by a series of 

 slender bones (d, Fig. 184), which are joined to the 

 spinous processes of the vertebrae, and are formed 



