158 THE PROGRESS OP DEGRADATIOX. 



termination of the abdominal cavity, and mark the line of 

 separation between the vertebrae of the trunk (dorsal, lum- 

 bar, and sacral), and the third and last, or caudal division of 

 tlie column, — a division represented in man by but four ver- 

 tebrae, and in the crocodile by about thirty-five, but which is 

 found to exist, as I have already said, in all the more perfect 

 forms. The limbs, then, in all the symmetrical animals of 

 the first three classes of the vertebrata, mark the three great 

 divisions of the vertebral column, — the division of the neck^ 

 the division of the trunks and the division of the tail. Let 

 us now inquire how the case stands with the fourth and lowest 

 class, — that of the fishes. 



In those existing placoids that represent the fishes of the 

 earliest vertebrate period, the places of the double fins, — pec- 

 torals and ventrals, — which form in the ichthyic class the 

 true homologues of the limbs, correspond to the places which 

 these occupy in the symmetrical mammals, birds, and reptiles, 

 The scapular bases of the fore or pectoral fins ordinarily be- 

 gin opposite the twelfth or fourteenth vertebra;* but they 

 range, as in man and the mammals, in a forward direction, 

 so that the fins themselves are opposite the eighth or tenth. 

 The pelvic bases of the ventral fins are placed nearly opposite 

 the base of the abdomen, so that, as in all the symmetrical 

 animals, the vent opens between, or nearly between, those 

 hinder limbs which the bases support. In the rays, which, 

 so far as is yet known, did not appear in creation until the 

 Secondary ages had begun, the bases of the fore limbs, i. e. 

 pectoral fins, are attached to the lower part of a huge cervi- 

 cal vertebra, nearly equal in length to all the trunk vertebrae 

 united ; and in the Chimeridse, which also first appear in the 

 Secondary division, they are attached, as in the osseous fishes, 

 to the hinder part of the head. But in the representatives 



* The twelfth in Spinax Acanthias, and the fourteenth in ScylUum 

 Stellare. 



