78 Recherches sur les Poissons Fossiles. 



The circumstances requiring such an extraordinary form 

 of the bony skeleton of fishes, whatever they were during 

 the period of the coal formation, no longer exist in re- 

 gard to these animals. But we find the cartilaginous fishes 

 of the present day, the Sharks, the Rais, and Sturgeons, 

 present a similar peculiarity of structure in relation to the 

 caudal termination of the vertebral column ; and reason- 

 ing from the known habits of these species, and the man- 

 ner in which they employ the peculiarity in question, we may 

 be thus led to satisfactory conclusions as to the manner 

 in which it served the osseous fishes, whose remains are 

 found in the strata of the coal formation. 



In regard to their habits, suppose we divide fishes into such 

 as prey upon each other, or derive their food exclusively from 

 substances found in water, and such as obtain it partially at 

 least from insects not inhabiting water, and which conse- 

 quently require an effort to rise into the air. 



This latter habit would require a great development 

 either of the lower lobe of the caudal, or of the anal fin, as we 

 see in the Perilamps, or fishes of our own day, which feed 

 exclusively on insects. On the other hand, the peculiarity of 

 fossil fishes now in question is of an opposite character, and 

 only calculated for plunging to greater depths beneath water, 

 and for violent struggles within that element. 



From this view of such structure w^e may conclude, that 

 aquatic insects were rare during the period of the coal 

 formation, although the remains of Arachnidans in these 

 strata, would render the existence of land insects probable. 

 This indeed, is in strict harmony with the great scarcity of 

 fossil insects in the rocks of the coal formation. 



M. Agassiz remarks, that besides other peculiar charac- 

 ters which the family of Lepidoides possess in common, there 

 are some with the caudal fin inserted obliquely, as in the fore- 

 going figure, and others in which it is inserted on a symmetri- 

 cal base, as in fig. 2, the former he names Heterocerous, 



