416 MANUAL OF ZOOLOGY. 



(Lepidostens] inhabits the rivers and lakes of North America, 

 and attains a length of several feet. The body is entirely 

 clothed with an armour of ganoid scales, arranged in obliquely 

 transverse rows. The vertebral column is exceedingly well 

 ossified, and is reptilian in its characters, the bodies of the 

 vertebrae being "opistboccelous." The jaws form a long 

 narrow snout, armed with a double series of teeth ; and the 

 tail is heterocercal. 



The Polypteri, of which several species are known, inhabit 

 the Nile, Senegal, and other African rivers, and are remarkable 

 for the peculiar structure of the dorsal fin (fig. 175, 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 stated to possess external bran- 

 chiae when young, losing them when fully grown. This 

 observation, if confirmed, will bring the Ganoids into a 

 nearer relationship with the Mud -fishes (Lepidosiren). Another 

 group of Lepadoganoids is formed by the Trout-like Amice of 

 the fresh waters of the United States, in which the scales are 

 rounded and overlap one another, the tail is homocercal, and 

 the vertebral column is ossified. 



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 protected by large ganoid plates, 

 which in many cases are united together at their edges by 

 sutures. The tail is heterocercal. 



The family Sturionida comprises the various species of Stur- 

 geon, which are found in the North, Black, and Caspian seas, 

 whence they ascend the great rivers for the purpose of spawn- 

 ing. Other allied forms are peculiar to the North American 

 continent (e.g., the Paddle-fish, Spatularia). The vertebral 

 column in the Sturgeon remains permanently in an embryonic 

 condition. The notochord is persistent, and the vertebral 

 centra are wanting, but the neural arches of the vertebrae reach 

 the condition of cartilage. The mouth is destitute of teeth, 

 and the head is covered with an armour of large ganoid plates 

 joinecj together at their edges by suture. Rows of detached 

 ganoid plates also occur on the body. The various species of 



