XX] OSTEICHTHYES 473 



understand the "holostylic" modification of the skull, for it is an 

 obvious advantage to have the upper jaw firmly fixed if the fish has 

 to bite firmly and strongly. .The loose hyostylic arrangement on the 

 contrary is connected with mobility in the upper jaw, which indicates 

 a capacity to enlarge the gape to a great extent and thus to swallow 

 prey whole. 



Sub-class II. OSTEICHTHYES. 



The Bony Fish constitute an enormous group; they include over 

 10,000 species as against about 400 species of Chondrichthyes. Of 

 these 10,000, the overwhelming majority though divided into 

 families differing in minor points are obvious modifications of one 

 well marked type with which we shall become better acquainted 

 immediately, and which is known as the Teleostean type. There 

 remain, however, about a dozen genera including twenty or thirty 

 species which differ from this type in important characters. These 

 aberrant fish are in fact, highly interesting survivals of more 

 primitive types of fish which have almost died out. 



By the study of their structure we obtain some light not only on 

 the manner in which the bony fish was evolved from a cartilaginous 

 fish but also on the manner in which the most primitive type of 

 land animal was derived from a fish. 



For this reason a disproportionate amount of attention must be 

 given to these aberrant forms. 



They are divided into five orders, viz. 



(1) the Aetheospondyli including one genus Lepidosteus ; 



(2) the Protospondyli including one genus Amia; 



(3) the Chondrostei with four genera viz. Acipenser, Scophi- 

 rhynchus, Polyodon and Psephurus. 



(4) The Polypterini including two. genera viz. Polypterus and 

 Calamoichthys. 



(5) The Dipnoi including three genera Ceratodus, Lepidosiren, 

 and Protopterus. 



All other Osteichthyes are included in one order called the 

 Teleostei. 



The Aetheospondyli, Protospondyli, Chondrostei, and Polypterini 

 were formerly grouped together as Ganoids because some of them 

 possess hard scales with a covering of shining material termed 

 ganoin. The unsuitable character of the name is apparent when 

 we reflect that these scales are found only in four out of the nine 

 genera which constitute Ganoidei. 



