VERTEBRATA: FISHES. 



493 



Senegal, and other African rivers, and are remarkable for the peculiar 

 structure of the dorsal fin (fig. 265, A), which is broken up into a number 

 of separate portions, each composed of a single spine in front, with a soft 

 fin attached to it behind. They belong to the Crossopterygious Ganoids, 

 in which the pectoral' fins always, and the ventral fins often, consist of a 

 central lobe or stem, which is covered with scales, and to the sides of 

 which the fin-rays are attached. Two species of Polypterus have recently 

 been shown to possess external branchiae when young, losing them when 

 fully grown. Calamoichtkys, also from the rivers of Africa, resembles 

 Polyplerus in most respects, but the body is serpentiform, and there are no 

 ventral fins. Another group of Lepidoganoids is formed by the Trout- 



Fig. 266. A, Lepidosteus osseus, the "Gar-Pike" of the American Lakes. B, Aspi- 

 dorhynchiis, restored (after Agassiz) ; a Jurassic Ganoid allied to Lepidosteus, but 

 having a homocercal tail. 



like Amice of the fresh waters of the United States, in which the scales 

 are rounded and overlap one another, the tail is slightly heterocercal, and 

 the vertebral column is ossified. The air-bladder in Amia is subdivided, 

 and can be used as a functional respiratory organ. 



The section Placoganoidei includes the largest and best known of all the 

 living Ganoid fishes namely, the Sturgeons and it also contains some 

 highly singular fossil forms. The sub-order is defined by the fact that 

 the skeleton is always imperfectly ossified, and often retains the notochord, 

 whilst the head and more or less of the body are usually protected by 

 large ganoid plates, which in many cases are united together at their edges 

 by sutures. The tail is heterocercal. 



The family Chondrosteidce, or Sturionida, comprises the various species 

 of Sturgeon, which are found in the seas of the northern hemisphere, 

 whence they ascend the great rivers for the purpose of spawning. The 

 vertebral column in the Sturgeon remains permanently in an embryonic 

 condition. The notochord is persistent, and the vertebral centra are want- 

 ing, but the neural arches of the vertebrae reach the condition of cartilage. 

 The mouth is destitute of teeth, and the head (fig. 253, B) is covered with 

 an armour of large ganoid plates joined together at their edges by suture. 

 Rows of detached ganoid plates also occur on the body. The various 

 species of Sturgeon attain a great size, one the Beluga often'.measuring 

 twelve or fifteen feet in length. They are commercially of considerable 

 importance, the swimming-bladder yielding most of the isinglass of com- 



