82 VEETEBEATA. 



attacks these primitive sharks were thus defended. An Ichthyolite has 

 been discovered in Indiana, claimed to be Silurian, but more probably 

 Devonian. According to Dana, the earliest fish remains in America 

 occur in the Schoharie grit. As the Cartilaginous Fishes dwindled, 

 the Teleosts took their place. They began in the Cretaceous, but 

 abounded most in the Tertiary deposits. Respecting fossil Fishes, 

 Agassiz says: "I have not yet found a single species which was per- 

 fectly identical with any marine existing Fish, except the little Capel- 

 in (Mallotus villosus), which is found in the nodules of clay of un- 

 known age in Greenland." "The Scombcroids" (the Mackerel tribe), 

 observes Owen, " seem now to be at the head of the piscine modifi- 

 cation of the Vertebrate series." The ancient Placoids and Ganoids, 

 however, were more highly organized than the modern Cycloids and 

 Ctenoids, and manifested a closer affinity to the air-breathing, cold- 

 blooded Class. 



Order 1 — Ganoids. 



The Ganoids have a partially ossified frame, rhomboidal or angular 

 scales, and labyrinthoid teeth. No living Ganoid has been observed in 

 the Southern Hemisphere. 



Of fossil Ganoids, 80 are found in the Devonian, 61 in the Carbon- 

 iferous, 22 in the Magnesian Limestone, 23 in the Trias, 100 in the 

 Lias, 154 in the Oolite, 36 in the Chalk, and 29 in the Tertiary. After 

 the Triassic Period, they lost the Pakeozoic feature of heterocercal tails. 

 The Devonian species have many reptilian characters ; they are often 

 found in nodules of a flattened, elliptic form, such nodules being created 

 by the oil of the decomposing Fish rendering the surrounding sand 

 compact. The Placoganoids, which first appeared in the Upper Si- 

 lurian, died out in the Carboniferous. The Lepidoganoids ranged from 

 the Devonian to the Tertiary. Evidence of a true Sturgeon has not 

 been met with below the Eocene clay of Sheppey. The Oolitic Fishes 

 are generally homocercal Ganoids. 



