22 



FOSSIL FISHES. 



They are divided into two groups; Heterosiraci, which have no 

 bone-cells in their shields, viz. -.—Scapliaspis, Pteraspis, and 

 Gyathaspis; and Osteostraci, viz.:— Cephalaspis, Auchenaspis, and 

 JEuleraspis, in which hone-cells are present. 



Cephalaspis Lyelli, from the Old Red of Forfar, is one of the 

 most perfect of these older fish. (See Figs. 30 and 31.) 



Sub-order III. — Drpfloi. 



The Dipnoi (Table-case 36, and portion of Wall-case 5), form a 

 very peculiar sub-order, having three living representatives, namely, 

 Protopterus, in Africa (Fig. 35), Lepidosiren, in South America, and 

 Ceratodus, in Australia (Fig. 32). 



Fig. 32.—" The Australian Mudfish," Ceratodus Forsteri (recent), Australia. 



In these fishes the skeleton is notochordal, there are a pair of 

 pterygopalatine teeth, and a pair of incisor-like vomerine teeth 

 above, and a third pair of teeth in the lower jaw. There are two 

 pairs of nostrils, more or less within the mouth, and the air-bladder, 

 single or double, takes on the function of a lung. There is one 

 external branchial aperture into which the gills project freely, as in 

 Ganoids and Teleosteans. 



The caudal fin is diphycercal or heterocercal, the scales are 



Fig. 33. — Diptertu tnacrolepidotue, Sedgw. and Murch., Old Red Sandstone, Scotland. 

 p, Pectoral ; v, ventral ; c, anal ; (21, anterior dorsal ; d2, posterior dorsal ; c, caudal fin. 



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 Fig. 33.) 



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



