38 



FISHES ABUNDANT 



creatures, zoophyta, polypiaria, conchifera, Crustacea ; but 

 to these are added numerous fishes, some of which are oi 

 the most extraordinary and surprising forms. Several of 

 the strata are crowded with remains of fish, showing that 

 the seas in which those beds were deposited had swarmed 

 with that class of inhabitants. The investigation of this 

 system is recent ; but already* M. Agassiz has acertained 

 about twenty genera, and thrice the number of species. 

 And it is remarkable, that the Silurian fishes are here only 

 represented in genera; the whole of the species of that 

 era had already passed away. Even throughout the sub- 

 groups of the system itself, the species are changed ; and 

 these are phenomena observed throughout all the subse- 

 quent systems of geological eras ; apparently arguing that, 

 during the deposition of all the rocks, a gradual change of 

 physical conditions was constantly going on. A varying 

 temperature, or even a varying depth of sea, would at 

 present be attended with similar changes in marine life ; 

 and by analogy, we are entitled to assume, that such vari- 

 ations in the ancient seas might be amongst the causes of 

 that constant change of genera and species in the inahabi- 

 tants of those seas, to which the organic contents of the 

 rocks bear witness. 



Some of the fossils of this system — the cephalaspis, 

 coccosteus, pterichthys, holoptychius — are, in form and 

 structure, entirely different from any fishes now existing, 

 only the sturgeon family having any trace of affinity to 

 them in any respect. They seem to form a sort of con- 

 necting link between the Crustacea and true fishes. 



The cephalaspis may be considered as making the small- 

 est advance Irom the crustacean character ; it very much 

 resembles in form the asaphus of lower formations, having 

 a longish tail-like body inserted within the cusp of a large 

 crescent-shaped head, somewhat like a saddler's cutting- 

 knife. The body is covered with strong plates of bone, 

 enamelled, and the head was protected on the upper side 

 with one large plate, as with a buckler — hence the name, 

 implying buckler-head. A range of small fins conveys 

 the idea of its having been as weak in motion as it is 

 strong in structure. The coccosteus may be said to mark 

 the next advance to fish creation. The outline of its body 

 is of the form of a short thick coffin, rounded covered 

 with strong bony plates, and terminating in a long tail 

 * June, 1840. 



