66 VERTEBKATA. 



Ganoids, and hence all have cartilaginous skeletons, and verte- 

 brated or heterocercal tails Many, however, were remarkable 

 for their heavy bone armor. 



During the Carboniferous the sharks were predominant, over 

 one hundred species being known from the lower Carboniferous, 

 but in the Mesozoic they were under subjection to the huge rep- 

 tiles. The largest sharks lived during the Tertiary, the Car- 

 charodon niegalodon being estimated at fifty to sixty feet long. 



The Teleosts, the characteristic modern fishes, appeared in the 

 Cretaceous period, and modern types abounded in the Tertiary. 



About 9,000 species of fishes are now living, of which only 

 about one hundred are Ganoids (seven genera), and four (three 

 genera) are Dipnoi. However, one genus {Ceradotus), of the 

 latter class has survived from the Triassic, and the sharks have 

 endured all geologic changes since the Silurian, and seem to be 

 enjoying themselves to-day. 



OKDER DIPNOI. 



The members of this small order have so many features in 

 common with the Amphibia that the oldest known form {Lejpi- 

 dosireii) was regarded as the lowest of that order. They have 

 no ossified vertebrae, but a persistent notochord ; the limbs are 

 jointed paddles, or filiform bodies ; the heart has two auricles ; 

 and in addition to gills they possess true lungs, whence the name 

 " Lung Fishes." 



The discovery, since 1S70, of living individuals of the Triassic 

 genus Ceratodus, enabled ichthyologists to trace relationship with 

 some of the early ganoids, and Dr. Gunther, on account of this 

 affinity has placed the Dipnoi in the order Ganoidei. The De- 

 vonian fishes, Dinichthys^ Dipterus and Holoptychius, were early 

 members of the group. 



The living species are Ze^^c?05^re7^^a/"(26?o«a, from the Ama- 

 zon, Protopterus annectens of Africa, and Ceratodus Fosteri and 

 C. miolepisy of Australia. 

 No. 104. [340] Ceratodus . 



Two Teeth. (Cast). The teetli of this Dipnoan are puzzling from the 

 variety of shapes they assume. They have in general an uneven or undulating 

 upper surface of dentine and enamel, and an under-layer of reticulated osseous 

 tissue. The genus is found by Cope in Upper Carboniferous of Illinois, and 

 by Marsh in the Jurassic of Wyoming. These specimens are from the Upper 

 Tyias (Keuper) at Stuttgardt, Wurtemberg, and are now in the Ward Collec 

 tion of the University of Rochester. 



