332 



AcirK.N'sKiti;).]'; 



Order II. CHONDROSTEI. 



Skeleton partly cartilaginous. Skin with osseous 

 bucklers or naked. Caudal fin heterocercal, with 

 fulcra. 



Chondrostui, Miillcr, Abhundl. Ak. Wis^. Berlin, 1844, p. 147. 

 Placoganoidei, Orcen, Compar. A Hat. of Vcrteb): i. p. 12. 



Seas and rivers of the temperate and arctic regions of the northern 

 hemisphere. 



Fam. 1. ACIPENSKRIIX^. 



Acipensevini, Midler, Abhandl. Ak. Wiss. Berlin, 1844, p. 147. 



Body elongate, subcylindrical, with five rows of os- 

 seous bucklers. Snout produced, subspatulate or coni- 

 cal, with the mouth at its lower surface, small, trans- 

 verse, protractile, toothless. Nostrils double, in front of 

 the eye. Four barbels in a transverse series on the 

 lower side of the snout. Vertical fins with a single 

 series of fulcra in front. Dorsal and anal fins ap- 

 proximate to the caudal, which is heterocercal. Gill- 

 membranes confluent at the throat, and attached to 

 the isthmus. Branchiostegals none. Gills four • two 

 accessory gills. Air-bladder large, simple, commu- 

 nicatin£2r with the dorsal wall of the cesophaarus. 

 Stomach without blind sac. Pancreas divided into 

 pyloric appendages. Rectum with a spiral valve. 



Inhabitants of the seas of the temperate and arclic regions of the 

 northern hemisphere, periodically entering rivers to pr^nagato. 

 Some species entirely confined to fresh water. 



Two genera : — 

 The rows of osseous bucklers not confluent. . 1. Acipensir. p. .'}.;.■) 

 The rows of osswus bucklers eontluent on thi' tail. 



2. Si(ij)hirln/)td(iis. p. '-Mo 



