Fishes of the Western A^orth Atlantic 2 7 



in young individuals, but becoming progressively blunter with age or even disappearing 

 through absorption. Skin between scutes with small ossifications. Snout protruding. 

 Mouth inferior, protractile. Teeth absent in adults. Barbels 4, in a crossrow in 

 front of mouth. Gills 4, and an accessory opercular gill. Branchiostegals absent. 

 Gill rakers fewer than 50. Opercle absent. Head covered by bony plates separated 

 by sutures; particularly visible in younger specimens. Dermal skeleton without 

 ganoine. Caudal fin with typical fulcra. Dorsal and anal fins behind ventrals. Pec- 

 toral fin with first ray enlarged and ossified. Tail heterocercal. Air bladder large, 

 simple. Stomach with numerous pyloric appendages, forming a compact and rather 

 large gland. Rectum with spiral valve. 



Anadromous and freshwater fishes of the northern hemisphere; Upper Cretaceous 

 to Recent. 



Remarks. The cartilaginous skull has an opening under the frontal and parietal 

 plates, usually completely roofed over by these membrane bones. Hence there is a cav- 

 ity, evident in sagittal section of the head, for which we propose the nzme frontal sinus. 



In young specimens of A. oxyrhynchus of America and A. sturio of Europe, there 

 is a membranous, elongated, nonossified area on the top of the head which Ryder^ called 

 the "fenestra" or "fontanelle." In specimens less than 40 inches long, this can usually 

 be found by probing with a needle between the frontal and parietal plates; thus a probe 

 can be passed downward into xht frontal sinus. Adults, and in rare instances young, have 

 the plates fully joined, closing the fontanelle. 



The protractility of the mouth is made possible by movements of the jaws and 

 palatoquadrate bone {10: 409). 



The pseudobranchs are rudimentary In Scaphirhynchus (jj: 26) and completely 

 lacking in Pseudoscaphirhynchus {li: 104). The large element of the gill cover repre- 

 sents the subopercle, not the opercle (jo: 48—50). 



The appearance of the Sturgeon changes considerably with age; the snout 

 becomes shorter and blunter and the shields smoother while the lower caudal 

 lobe, not fully formed' in very small specimens, becomes longer. Apparently the 

 shields may be partially or completely absorbed in connection with ripening of the 

 gonads. 



Genera. The family Acipenseridae is divided into two subfamilies. Acipenserini 

 (true Sturgeons), with spiracles present, is represented by two genera: Huso, Lower 

 Pliocene to Recent, with two species; and Acipenser, Upper Cretaceous to Recent, 

 with about 16 species. Scaphirhynchini (Shovelnose Sturgeons), without spiracles, 

 also includes two genera: Scaphirhynchus, with two species;* and Pseudoscaphirhynchus, 

 with three species. 



6. Ryder was probably the first American author to describe the presence of a fontanelle in A. oxyrhynchus {65: 233); 

 for that in A. sturio, see Stannius {68: 26) and Antoniu {4: 113, 114). 



7. Grote, et at. {36: 215, fig. 158) and Roule {62: pi. 4, fig. 7) have presented drawings of young A. sturio in which 

 the lower lobe is not well grown as yet. 



8. Although one of the American species has been described as a separate genus, Parascaphirhynchus [31: 37-44), 

 recent studies fail to substantiate this (S: 174). 



