72 GENERAL RELATIONS OF FOSSIL FISHES. 



larger proportion of those existing at a subsequent time, as at 

 the present epoch, belong to the two orders with horny scales, 

 which comprehend at least three-fourths of the 8000 living 

 species of Fishes known to Naturalists. Of the fossils of these 

 last orders, which abound in the formations of the Tertiary 

 period, it is sufficient to say, that they may nearly all be referred 

 to families which have been described as now existing ; that 

 many of them belong to the same genera with recent Fishes ; 

 but that it is doubtful if any of them are of the same species with 

 those now tenanting our seas. 



591. The sketch here given of the principal groups of Fossil 

 Fishes, is sufficient to illustrate some very interesting points in 

 the history of this class. In the first place, we see that the first- 

 created Vertebrated animals so far resembled the Invertebrated 

 classes, as to possess a very dense external skeleton ; whilst their 

 internal skeleton was so soft, as not to be capable of being pre- 

 served ; and it appears to have been to the Crustacea, which are 

 among the highest of the Articulated series, that these Fishes 

 were most nearly related. On the other hand, among the Fishes 

 of the present time, the Cyclostomata, which present the cha- 

 racters of the Vertebrata in their most imperfect form ( 585), 

 are rather analogous to the Annelida^ or Worm tribe ; with which, 

 indeed, some of them were actually classed by Linneeus. We 

 may next observe, that the covering of dense enamelled scales, 

 in which all the Fishes of the earlier formations were inclosed, 

 rendered necessary a peculiar conformation in the mouths of 

 those, which were destined to prey upon them and to restrain 

 their multiplication. Thus we find the mouths of the predaceous 

 Ganoid fishes, such as the Holoptychius and its allies, and in 

 those of the Cestracionts and Hybodonts, a pavement-like cover- 

 ing of flat enamelled teeth, adapted to crush the hardest sub- 

 stances. But when the enamelled-scaled Fishes had given place 

 to those furnished with a softer covering, we find these enamelled 

 pavements of the jaws replaced by sharp cutting teeth, the 

 predaceous Ganoids disappearing altogether, and the Cestracionts 

 and Hybodonts giving place to the Sharks. What was the 

 purpose for which all the earlier races of Fishes were provided 



