146 



THE PLACOID BRAIN. 



when found in the mature animal, evidences of a high, not 

 of a low standing, on what principle, I ask, are we to infer 

 that the peculiarity of a heterocercal tail, embryonic in the 

 salmon, is, when found in the mature placoid, an evidence, 

 not of a high standing, but of a low 1 Every true analogy 

 in the case favours an exactly opposite view. In the hetero 

 cereal or one-sided tail, the vertebral joints gradually dimi- 

 nish, as in the tails of the Sauria and Ophidia, till they termi- 

 nate in a point ; whereas the homocercal tail common to the 

 osseous fishes exhibits no true analogy with the tails of the 

 higher orders. Its abruptly terminating vertebral column, 

 immensely developed posterior processes, and broadly ex- 

 panded osseous rays, seem to be simply a few of the many 

 marks of decline and degradation which fishes, the oldest of 

 the vertebrata, exhibit in this late age of the world, and which, 



Fig. 54. 



a. Tail of Spinax Acanthias. 



b. Tail of Ichthyosavrus Tenuirostris (Buckland), 



