72 



FISH GALLERY. 



Sturioniformes (Sturgeons). 



Tde Sturioniformes or Chondrostei are the fishes long known as 

 the " Cartilaginous Ganoids." In the most ancient forms (e. g. 

 Palceoniscus, 196, and fig. 42) the covering of the body consists 

 of a continuous cuirass of rhombic scales, rarely of cycloid scales. 

 In the modern forms the skin is almost devoid of scales, as in 



Fig. 42. — Restoration of Palaoniscus macropomus. 

 (After E.H. Traquair.) 



Pala?- 

 oniscus. 



Polyodon, the Paddle-fish (211), or there are five longitudinal rows 

 of keeled scutes, as in Acipenser, the Sturgeon (201). The endo- 

 skeletonis largely cartilaginous; the head is covered with bony plates 

 of dermal origin ; the notochord is persistent and not constricted by 

 the formation of vertebral centra. There is a single dorsal fin 

 and a single anal fin, with fulcra, i. e. spine-like scales along the 

 front edge ; the fin-rays of the median fins are more numerous 

 than the basal elements ; the caudal fin is usually heterocercal and 

 with fulcra. The pelvic fins have a series of basal cartilages. An 

 infraclavicle is present in the pectoral girdle, in addition to the 

 clavicle and supraclavicle (see 208). The teeth are small or 

 absent. The range of the suborder is from the Lower Devonian 

 to the present time. 



The Palseoniscidfe (196-197) are extinct fishes with elongate 

 and fusiform body, with short-based dorsal and anal fins, and with 

 a complete investment of rhombic scales, rarely with cycloid scales. 

 The Palaeoniscid fishes range from the Devonian to the Jurassic, 

 and were most abundant in Carboniferous and Lower Permian 

 times. Cheirolepis (197), is of Devonian age, and Palaoniscus 

 (196) Upper Permian. 



