120 PICTURESQUE SKETCHES 



while they differ from those of earlier date in one 

 point already alluded to, are not unlike them in some 

 respects. The difference consists in the non-pro- 

 longation of the back-bone or vertebral column of the 

 fish into the upper part of the tail-fin, a condition 

 that obtains with regard to all known species, without 

 exception, obtained from the older rocks ; while the 

 termination of the vertebral column before the tail-fin 

 commences characterises those of newer date. With 

 the exception of this difference, of which it is not 

 easy to conjecture the exact meaning, there was con- 

 siderable analogy and much general resemblance of 

 structure. 



We may consider the fishes of these rocks as form- 

 ing three principal groups. First, those with power- 

 ful crushing teeth in the palate, comprising animals 

 which were probably for the most part slow swim- 

 mers, and feeders on the encrinites, crustaceans, and 

 shell-fish, in the shoals and near shore ; secondly, 

 those with sharp teeth, of small size, encased like the 

 former in boxes of bone coated with enamel, the 

 representatives in fact of the sauroid fish, which 

 played so important a part in the more ancient 

 seas ; and, thirdly, the great and voracious tribe of 

 sharks, coated merely with tough skin, dotted over 

 with points of enamel, and swimming rapidly and 

 freely in the open seas in search of prey. 



There is nothing of importance in the earlier forms of 

 the two latter groups, and they will be more properly 

 considered when we come to the description of the in- 

 habitants of the sea during the deposit of the newer 

 rocks. Of the peculiar apparatus provided for the for- 

 mer, the annexed wood-cut (46) will give some idea. It 



