20 



Fossil Fishes. 



Table-case, 

 No. 36. 



external branchial aperture into which the gills project freely, as 

 in Ganoids and Teleosteans. 



The caudal fin is diphycercal or heterocercal, the scales are 

 cycloidal ; the paired fins are acutely-lobate, with a central jointed 

 cartilaginous stem fringed with radial cartilages and dermal fin-rays. 



Dipterus, from the Devonian rocks, has two dorsal fins, a hetero- 

 cercal tail and one anal fin. The head is covered with bony ganoid 

 plates, and the scales are also ganoid. {See Figs. 43, 45.) 



Fig. 44. — Mouth of Ceratodus. nn, narial openings ; x, vomerine teeth; xx, palato- 



pterygoid teeth ; xxx, mandibular teeth. 



Fig. 45.— Jaws of Dipnoan Fishes; 1, upper and lower jaws of Dipterus; 2, 3, upper 

 and lower jaws of Ctenodus ; 4, upper jaw of Palcedaphus. 



Ctenodus, a common Carboniferous form, has bony head-plates 

 like those of Dipterus, but the scales are thin and the tail probably 

 diphycercal. Teeth indistinguishable in character from the modern 



