49 2 MANUAL OF ZOOLOGY. 



V. There is always a swim-bladder, which is often divided 

 by partitions into several cells, and is always connected with 

 the gullet by an air-duct, as in the Malacopterous division of 

 the Teleostean fishes. In Polypterus the air-bladder is double 

 and sacculated. 



VI. As to the structure of the heart, the Ganoids differ from 

 the Bony Fishes, and agree with the Sharks and Rays in having 

 a rhythmically contractile bulbus arteriosus, which is furnished 

 with a special coat of striated muscular fibres, and is separated 

 from the ventricle by several rows of valves. This is a decided 

 advance in structure, as in this way the arterial bulb is enabled 

 to act as a continuation of the ventricle. 



VII. The intestine is often furnished with a spiral redupli- 

 cation of its mucous membrane, forming a spiral valve, such 

 as we shall afterwards see in the Sharks and Rays. 



The classification of the Ganoid fishes has hitherto proved 

 a matter of extreme difficulty ; and probably no arrangement 

 that has been as yet proposed can be regarded as being, in all 

 its details, more than provisional. A convenient primary 

 division is that into Lepidoganoids, in which the body is 

 furnished with scales of moderate size, and the endoskeleton 

 is generally more or less perfectly ossified, and Placoganoids, 

 in which the skeleton is imperfectly ossified, and the head and 

 more or less of the body are protected by large ganoid plates, 

 which in many cases are united together by sutures. Accept- 

 ing this division, the order Ganoidei may be divided into the 

 following sub-orders : 



SECTION i. LEPIDOGANOIDEI. 



Sub-order A. Amiadee. 



it B. Lepidosteidcz. 



ii C. Crossopterygidce. 



M D. Acanthodidce. (Extinct.) 



SECTION 2. PLACOGANOIDEI. 



Sub -order E. Ostracostei. (Extinct.) 

 it F. Chondrostddcz. 



The best known living fishes belonging to the Lepidoganoids are the 

 Bony Pike and the Polypterus. The Bony Pike (Lepidosteus, fig. 266, A) 

 inhabits the rivers and lakes of North America, and atttains 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 " opisthocoelous." 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, 



