GANOIDEI. 333 



characteristic of the genus. These were in the form of two 

 long curved spines, somewhat like wings, covered by finely- 

 tuberculated ganoid plates. From their form they cannot 

 have been of much use in swimming ; but they probably, as 

 suggested by Owen, enabled the animal to shuffle along the 

 sandy bottom of the sea, if stranded at low water. All the 

 species of Pterichthys are exclusively confined to the Old Red 

 Sandstone. 



Fig. 294. Pterichthys comutus. Old Red Sandstone. 



SUB-ORDER G. STURIONID^E. In this sub-order the skeleton 

 is almost altogether cartilaginous, and the notochord is per- 

 sistent. The exoskeleton is usually in the form of large gan- 

 oid plates, which are united into a shield over the head, but 

 are detached over the body. Sometimes the exoskeleton is 

 almost absent, and in no case is the mouth furnished with 

 teeth. The tail is heterocercal. 



This sub-order comprises the living Sturgeons, and is not 

 known with certainty to have corne into existence before the 

 Eocene Tertiary, where it is represented by the Acipenser toli- 

 apicus of the London Clay. In the Lias, however, occur two 

 species of the singular genus Chondrosteus, which have usually 

 been referred here, and have been regarded as being most 

 nearly allied to the Paddle-fishes (Spatularici] of North America. 

 The skull, however, is more completely ossified than is the case 

 with any living members of the Sturionufa; and the true place 

 of Chondrosteus must be regarded as uncertain. 



