332 



ACIl'ENSIiltllI.E. 



Order II. CHONDROSTEI. 



Skeleton partly cartilaginous. Skin witli osseous 

 bucklers or naked. Caudal fin heterocercal, with 

 fulcra. 



Chondrostei, Miiller, Abhandl. Ak. Wiss. Berlin, 1844, p. 147. 

 Placogauoidei, Oicen, Compar. Anat. of T'crtebr. i. p. 12. 



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

 hemisphere. 



Fam. 1. ACIPENSb:RIlX^. 



Acipenseriui, Miilkr, Abhandl. Ak. TFiss. 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- 

 nicating with the dorsal wall of the oesophagus. 

 Stomach without blind sac. Pancreas divided into 

 pyloric appendages. Rectum with a spiral valve. 



Inhabitants of the seas of the temperate and arctic regions of the 

 northern hemisphere, periodically entering rivers to propagate. 

 Some species entirely confined to fresh water. 



Tiuo genera : — 

 The rows of osseous bucklers uot contiuent. . 1. Acipenser, p. 333. 

 The rows of osseous bucklers contiuent on th'.^ tail. 



2. Scap/iir/ii/nc/iKs, p. 345. 



