FISHES. 



95 



fishes had a short ami vertit'ally HattcMit'd hody, much like that of the chajtodonts of 

 the present time, but covered with rhomboid enamelled scales and peculiar dermal 

 ribs (called pleurolepida), which covered either the whole body or only the anterior 

 portion with a sort of lattice-work. The tail was either homocercal or heterocercal. 

 Three families, Platysomid^b, Pleurolepida, and Pycxodontida, containing 

 numerous genera, have been described, but do not need characterization here. Their 

 geological appearance corresponded with the order in which tliey are mentioned. 



Order V. — CROSSOPTERYGII. 



Only two living genera, Polijpterus and Calamoichthys, represent this order in the 

 existing seas, but in former times tiie genera were many ; these fossils are now arranged 

 in five families, while the two genera just mentioned constitute the sixth (Polypieri- 

 DM) of the order. All of these forms, recent and fossil, agree in the following char- 

 acters: the caudal fin is usually dijihycercal, the dorsal divided into either two 

 large or many small divisions, the pectorals and ventrals have a scaly axis, and fulcra 

 are absent from all. Tlie throat is protected by two large plates, to which occasionally 



¥lQ. &%.— Polyiiterus bichir, bichir. 



smaller lateral ones are added. The scales covering the body may be either thin and 

 cycloid in character, or thick and rhomboid, like those of most ganoids. Some of the 

 fossil forms, like Ctenodus and Dlpterus, approach the lung-fishes the most closely of 

 any members of the sub-class. 



The two living genera, as we have just said, belong to one family. They have 

 rhomboid scales, a long, many-divided, dorsal fin. The mouth is placed at the end of 

 the body, and above it are two barbels. In Polypterus, ventral tins placed far back 

 are present, but in the long and slender Calamoichthys none are jiresent. The best 

 known species is the bichir {P. bichir) of the upper Nile and other African rivers. It 

 reaches a length of about eighteen 

 inches, and is regarded as a valuable 

 food fish, in fact the best that the river 

 affords. In the system of Cuvier, the 

 bichir was placed among the bony 

 fishes, in the vicinity of the herrings. 

 One of the most interesting features 

 connected with the fish is that, in the 



Fic. 69. — Young bichir, showing ((7) external gill. 



young, external gills are present. Two other s])ecies, P. senegalensis and P. endlicheri, 

 are known. All live in the deejier pools, and ap]iarently l)ury themselves in the slime 

 and ooze on the bottom, where they feed on fishes and other aquatic animals. The 

 nasal cavity is very complicated, a labjrinth being formed by five parallel passages, 

 each of which contains a gill-like fold. Nothinc: similar is known in other fishes. Of 



