260 



THE GEOLOGIST. 



one or two feet in length. A few genera contained species 

 as much as three or four feet long, but none which could be 

 compared with the dimensions of the sword-fish, shark, &c. 

 of our own seas. M. Agassiz then remarked that there was 

 a singular uniformity of size in the species of almost all ge- 

 nera and families, not only amongst fish, but other classes, 

 which he illustrated by reference to the different orders of 

 mammals, reptiles, insects, &c. In examining the fish of 

 these deposits, considerable diversity of type will be seen, 

 and the species must necessarily be referred to a variety of 

 orders and families. 



1. Thus there are four genera, Ctenacanthus, Onchus, 

 Ctenoptychius, and Ptychacanthus, belonging to the order 

 Placoides, which are furnished with spiny rays in the dorsal 

 fins, similar to the great Ichthyodorulites of the coal and 

 oolitic formations. In the order of Ganoides, the genera 

 Acanthodes, Diplacanthus, Cheiracanthus, Cheirolepis, pre- 

 sented a separate group; for although they were covered 

 with enamelled scales, these scales were so small as to give 

 the appearance of shagreen. The manner in which the fins 

 were sustained by spiny rays, or, in the absence of those 

 rays, the positions of the fins themselves, served as cha- 

 racters for determining the genera. 



2. The genera Pterichtys, Coccosteus, and Cephalaspis, 

 formed a second group, in which the size of the head and the 

 large plates which cover it, and also invest a large portion 

 of the body, and the moveable wing-like appendages placed 

 at the sides of the head, give them a most extraordinary 

 appearance. The large bony and granulated plates of the 

 Coccosteus caused it to be referred to the Trionyx. The 

 crescent-shaped head of the Cephalaspis, and peculiar scales 

 resembling transverse articulations of the body, made it 

 easily mistaken for a genus of Trilobites. Another curious 

 point in the structure of these genera, was the association 

 of exterior bony plates, with a soft and cartilaginous verte- 



