170 SUB-CLASS GANOIDEI. 



Fam. 1. Acipenseridae. Sturgeons. With five rows of keeled plates 

 in the skin, elongated snout, small mouth without teeth, with four barbels 

 in front of the mouth, opercular and four double gills. Stomach without 

 blind sac. With fulcra on the dorsal lobe of the caudal fin. Dorsal and 

 anal fin with two rows of supports (axonosts and baseosts). Large fishes 

 in the seas and rivers of the northern hemisphere, feeding on small 

 animals and plants. Most are migratory (anadromous). Caviare is the 

 ovary of the sturgeon. Acipenser L. with spiracle. Scaphirhynchus 

 Heckel, without spiracles. Fossil remains rare ; an Eocene species 

 (A. toliapicus Ag.) is known, and isolated remains of scales from the 

 Upper Lias of Whitby (Gyrosteus) and from the Upper Chalk of Kent 

 (Pholidurus). 



Fam. 2. Spatulariidae. Skin with small isolated scales, tail scaled as 

 in sturgeons, snout prolonged into a thin flat blade ; without barbels ; 

 with spiracle, without hyoid gill ; gills, 4 ; gill-rakers long, in a double 

 series on each arch, except on the fifth, which has only one series. 

 Air-bladder cellular. Jaws with fine teeth in young individuals. Fresh- 



FiG. 96. Aciyenser ruthenus (after Heckel and Kner). 



waters of N. America and China. Polyodon Lac., Mississippi ; Pse- 

 phurus Giinther, Chinese rivers ; a fossil genus Crossopholis Cope, from 

 the Eocene of Wyoming. 



EXTINCT FAMILIES. 



Fam. 1. Chondrosteidae. Parietals and frontals paired ; near the 

 parietal a great squamosal. Jaw edentulous. Branchiostegal rays 

 present. Operculum small, sub-operculum large. Body naked ; the 

 dorsal lobe of the caudal fin with fulcra and covered with rhombic ganoid 

 scales. Chondrosteus Egerton, Lower Lias, England. 



Fam. 2. Belonorhynchidae. Trias, Lias, may be placed here. 



Fam. 3. Palaeoniscidae. Body elongated with rhombic, rarely cycloid 

 scales ; with operculum, sub-operculum and branchiostegal rays. In no 

 single genus are the osteological characters well known. Devonian to 

 Jurassic. Cheirolepis Ag., Devonian ; Palaeoniscus Blv., Upper Per- 

 mian ; Coccolepis Ag., Jurassic. 



Fam. 4. Platysomidae. Deep-bodied fishes from the Carboniferous 

 to the Permian ; very similar to the Palaeoniscidae. Cheirodus M'Coy, 

 Eurynotus Ag., Platysomus Ag., Benedeniits Traquair. 



The Catopteridae, Triassic, may be placed here. 



