2 6 Memoir Sears Foundation for Marine Research 



Myodome absent. Interopercular bone absent. Radials in dorsal and anal fins cartilagi- 

 nous. Vertebral centra absent, the notochord entirely unconstricted throughout its 

 length. Air bladder well developed, oval, with smooth inner surface, opening into 

 oesophagus through a short, wide duct. Intestine with well-developed spiral valve (see 

 Berg, J-o: 387). 



Habitat. Fresh water, or, if marine, entering fresh water to breed. 



Families. Of the two living families, the Acipenseridae with four genera is much 

 more important throughout the northern hemisphere than Polyodontidae, which is rep- 

 resented now by only two freshwater genera, each with a single species; Polyodon 

 spathula (Walbaum), with a nonprotractile mouth, is found in eastern North America,^ 

 whereas Psefhurus gladius (Martens), with a protractile mouth, inhabits China; one 

 family, Chondrosteidae, is extinct, being known only from fossils of the Lower Lias 

 to Lower Cretaceous periods. 



Key to Modern Families 



I a. Snout in front of eyes little if at all longer than remainder of head, subconical, 

 rigid, more or less flattened in some but not blade-like in shape; a transverse 

 series of four fleshy and conspicuous barbels close in front of mouth; mouth on 

 lower surface of head, protrusible ; jaws toothless in adult ; five longitudinal rows 

 of large bony shields (scutes) on body, the lateral row extending onto base of 

 caudal, the skin elsewhere between them rough with small dermal ossifications; 

 bony plates separated by evident sutures covering head; anterior edge of pectoral 

 fins with a stout ossified ray; upper lobe of caudal much longer than lower. 



Acipenseridae, below. 

 I b. Snout in front of eyes much longer than remainder of head, blade-like In form 

 and flexible in American species; no barbels in front of mouth; mouth lateral in 

 position, protrusible (in P. gladius) or nonprotrusible (in P. spathula) ; jaws with 

 minute teeth; no bony shields on body, the skin everywhere on trunk smooth to 

 the touch; head not covered with bony plates; no ossified ray in pectoral fins; 

 lower lobe of caudal at least as long as upper lobe in American species. 



Polyodontidae. Paddlefishes. 

 Fresh water of eastern North America 

 and China. 



Family ACIPENSERIDAE 

 Sturgeons 



Characters. Body elongate and fusiform. Scutes or bony shields in five rows: 

 one dorsal, two lateral, and two ventral ; all scutes very sharp and strongly developed 



5. According to Dr. R. D. Suttkus (personal communication, July 9, 1957), a specimen of P. spathula, 481 mm FL 

 and weighing 354 g, was taken in the brackish water of Vermilion Bay at Cypremont Point, Louisiana, on May 15, 

 1954, by Antoine Carrere while trawling for penaeid shrimp. 



