POSTERIOR EXTREMITIES. 57 



fin, 53% however numerous these may be, with the 

 exception of the first, which articulates directly 

 with the radius or upper bone. 



In regard to the bones of the hinder extremities 

 which shew themselves externally as the ventral 

 fins, the os innominata, the thigh, the tibia, and 

 the tarsus, are represented in fishes by a single bone, 

 80, usually of a triangular form, but more or less 

 complicated by processes and projecting plates. Its 

 posterior side affords attachment to the rays of the 

 ventral fins, 81, 82. In eels and others, in which 

 the ventral fins are wanting, this bone is also 

 absent. These posterior members or ventral fins, 

 much more variable in their position than the cor- 

 responding limbs of the mammalia, project some- 

 times in advance of, sometimes beneath, and some- 

 times behind the anterior or pectoral members. 

 The rays of both these pairs of fins are divisible 

 lengthways into halves, and with the exception of 

 the outer ventral ray of the Acanthopterygians 

 (which is spinous, 81), are almost always composed 

 of joints or articulations, becoming, however, more 

 solid at their base. 



Besides the fins now mentioned, as representing 

 the four external members of birds and quadrupeds, 

 there are other single fins placed vertically, and 

 which serve a fish somewhat in the same manner 

 as a vessel is served by her keel and helm. Of 

 these some, called dorsal, 75, are attached to the 

 back, others, situate beneath, between the tail and ab- 

 domen, are named anal, 75 a , while a fine expan- 

 sion which usually terminates the body, is termed 

 F 



