146 MEDICAL ZOOLOGY. 



scales, or perfectly naked. Tail apt to be heterocercal ; opercular gill in 

 many; several valves in the arterial trunk; optic nerves not decussating; 

 spiral valve in intestine ; swimming bladder and air tube ; skeleton generally 

 cartilaginous; ventral fins abdominal. (Thymus gland in sturgeon and 

 shark.) 



Acipcmer. Squaloid ; dorsal and anal fins opposite; pancreas conglom- 

 erate; one gill opening; no rays. "Upper jaw formed by palatine bones, 

 firmly united to the maxillary; intermaxillary rudimentary." (Storer.) 



Mouth toothless. Operculum surrounded by a semicircular fold of integ- 

 ument. Head more or less distinctly quadrangular or club-shaped, the bones 

 of the skin concealed by a number of superficial plates. Eyes and nares to 

 side of head. The latter are two in number on either side, superior rounded, 

 in'erior elliptical. Muzzle produced, furnished beneath with four barbels. 

 Malar bone (!) rightangular, outer half covered with bones of the exoskeleton 

 surrounding the posterior part of the orbital opening. Mouth transversely 

 oval, placed behind the eyes, upon the under side of the muzzle, in a depres- 

 sion. Under lip cleft in middle. Body long, covered with small scales or 

 plates, skin rarely smooth, furnished with five rows of dermal plates, com- 

 monly assuming a pentangular form. One row occupies the median line of 

 back, two others (one on either side) lie directly behind this, starting from 

 the region of shoulder-blade and extending along the lateral aspect of body; 

 and two (one on either side) extending from base of coracoid bones along the 

 ventral region. The dermal plates composing these lines are largest above, 

 smallest beneath. Anal fin near caudal. Ventral fin nearer anal than pectoral. 

 'Dorsal fin about opposite anal. Caudal fin heterocercal, unequally forked, 

 the upper portion being the larger. Branchial arches five. The swimming 

 bladder communicates with the stomach. Pancreas consisting of a single 

 mass. Intestine furnished with spiral folds. 



The sturgeon is a fresh water fish. It is found in the rivers of Northern 

 Europe, Asia, America, and Western Africa. It may at times, however, 

 ^descend to the sea. 



H. A. rubicundus, Les. Body ruddy; flat between eyes; lateral semi- 

 plates oblique 39 ; 4 feet long. Great Lakes, U. S. 



2. A. brevirostris, Mitch. Snout blunt and short; dorsal scutes 9-12; 

 lateral 23-29 ; length 2-5 feet. Eastern Rivers, U. S. 



3. A. oxyrhyncus, Mitch. Snout elongated, spatula-like, covered with 

 strong bony plates ; length 2-7 feet. Eastern Rivers, U. S. 



4. A. huso. Northern Russia. 



