2892 



THE BONY FISHES AND GANOIDS 



Allied Extinct 

 Families 



Of the four species, one is restricted to the Mississippi river system, while the 

 others inhabit the rivers of Central Asia; all being exclusively fluviatile in their 

 habits. 



The genera Chondrosteus and Belonorhynchus from the European L,ias 

 severally represent two families differing from all the modern sturgeons 

 in the absence of a median unpaired series of bones in the head shield, 

 and also in the possession of branchiostegal rays. In the latter family the tail is 

 diphycercal, and there are longitudinal series of bony plates on the body; whereas 



in the former the tail is heter- 

 ocercal, and the body is either 

 naked or with a small series of 

 scales on the upper lobe of the 

 tail; both being furnished with 

 teeth. 



The scaled types 

 Scaled Types ,. , . 



of this suborder 



are so utterly unlike the stur- 

 geons in external appearance 

 that it is only by a study of 

 their internal structure that 

 their true affinities have been 

 AN EXTINCT ACiPENSEROiD FISH (Platysomus'). determined. They are all ex- 

 (From the Magnesian Umestone.) tinct, and mainly characteristic 



of the Secondary period, their 



remains being especially common in the British Lias. In both of the two principal 

 families the tail is of the heterocercal type. In one family, as typified by the genus 

 Palteoniscus, the body is elongated fusiform, and the teeth are slender and conical or 

 straight. On the other hand, Platysomus represents a second family (Platysomatid<z~) , 

 in which the body is rhomboidal, and the teeth in the upper jaw mainly confined 

 to the pterygoid bones obtuse. In both groups the scales are of the ganoid 

 type. 



THE FRINGE-FINNED GANOIDS Order CROSSOPTERTGII 



The whole of the members of the subclass under consideration described in the 

 foregoing pages constitute one great order (Actinopterygii), characterized, as men- 

 tioned on p. 2702, by the fan-like structure of the paired fins, and frequently also of 

 the caudal fin; the scales being generally of the cycloid or ctenoid type. These 

 fishes form, indeed, the dominant group of the present day; whereas the group now 

 to be considered is represented only by two existing species referable to as many 

 genera, and is mainly characteristic of the earlier epochs of the earth's history, 

 being abundant even in the Devonian and Carboniferous epochs, since which time 

 it has been steadily decreasing in numbers. These fringe-finned ganoids, as they 

 may be called, have the paired fins lobate, with an internal longitudinal axis be- 



