206 GEOLOGY. 



Fig. 116. 



classification of fishes. Agassiz discovered that there is 

 such a relation between the form of the scale and the in- 

 ternal organization of the fish, that those which have sim- 

 ilar scales are similar in their general character. This 

 discovery is of great importance in studying the palaeon- 

 tology of fishes, because the scales are so often preserved 

 in the rocks when the other structures of fishes have per- 

 ished. It is on this account that while the classification 

 of Cuvier answers admirably for living forms, that of 

 Agassiz is altogether better for the fossils. The four or- 

 ders of Agassiz are as follows: 1. Placoids. The name 

 comes from a Greek woYd,plax, a broad plate. The skin 

 in this order is covered with broad, irregular plates of 

 enamel, as in the shark family. The skeleton is soft and 

 cartilaginous, much of the firmness of the animal depend- 

 ing upon the external covering, which may be considered 

 in part as an external skeleton. In Fig. 117, 1, is repre- 

 sented one of the plates, and at 2 one of the prickly tu- 

 bercles of the ray-fishes, which belong to the same order. 

 2. Ganoid. This term comes from ganos, splendor. 

 Fishes of this order are covered in a regular manner 

 with scales of horn or bone, having a thick outer layer 

 of enamel, which is hard and bright. Such a scale is fig- 

 ured at 3. The sturgeon and trunk-fish belong to this 

 order. 3. Ctenoid. This name, from kteis, a comb, in- 

 dicates scales, 4 and 5, which are toothed or jagged on 

 the posterior margin. The perch is an example of this 

 order. 4. Cycloid. The term comes from JcuJclos, a cir- 



