ROOM V. LEPIDOIDS. 425 



these are included the small Amblypterus, Palseoniscus, and 

 Platysomus, of the carboniferous and triassic formations. 



In the Lepidoids the importance of the character of the tail 

 is strikingly manifest; for M. Agassiz has shown that all the 

 heterocercal lepidoids, with but one exception, belong to the 

 strata antecedent to the Lias; while the homocercals first 

 appear in that formation, and continue to the tertiary deposits, 

 in which the entire family disappears. 



The Lepidoids have an osseous skeleton, and are armed with 

 angular rhomboidal scales, parallel with the body. In some 

 species, the dental apparatus consists of rows of broad teeth, 

 in others of obtuse round teeth. The tail is relatively feeble. 



LIGX. 85. AMBLYPTERUS; OF THE COAL. 

 a. The heterocercal tail. ( not. size.) 



Amblypterus. Wall-case A. This small fish, from the 

 carboniferous shale, is an example of a characteristic hetero- 

 cercal lepidoid of that formation, to which six or seven 

 species belong. It is distinguished by the wide and many- 

 rayed fins without marginal rays, and the short and thick 

 tail. 



Palceoniscus. The fishes of this genus have a general 

 resemblance to the Amblypterus, but differ in the relatively 

 moderate size of the scales, and the numerous little rays on 

 their margins. There are upwards of twenty-five species; 

 sixteen of which belong to the Coal formation, and these have 

 smooth scales, with the exception of four species that occur 

 in the strata at Burdie House, in which the scales are 

 striated and punctated, like the seven or eight species of the 



