CLASSES, ORDERS, ETC. 63 



orown-olive, glossy ; umbones straight, excentric; umbonal region 

 compressed, rugose ; ligament short, prominent, forming an angle 

 with the lower margin ; upper margin curved and raised into a 

 sort of a crest, length 2^ inches ; breadth 3! inches. Small rivers, 

 canals, and ponds. A. anatina. 11 



Family III. Dreissenidae. 



Shell boat-shaped, equivalve, furnished with a byssus ; umbones 

 placed at the extreme end ; hinge with small teeth or edentulous 

 ligament internal. 



5. Dreissena. 



(a) Shell mussel-shaped, triangular, keeled in the centre of 

 both valves, olive or yellowish-brown, marked transversely with 

 zigzag streaks of purple or dark brown ; ligament long, narrow, 

 and fitting into a groove in the hinge of both valves ; umbones 

 incurved, small, placed at the anterior end ; lower margin incurved, 

 length i to 1 1 inch. Canals, lakes, and rivers. 



D. polymorpha. 



CLASS II. MALACOZOA GASTROPODA. 



Shell univalve or none. Body with a distinct head, and two 

 or four tentacles ; eyes situated at the extremity of the dorsal 

 tentacles or at the base of them ; respiration effected by gills or 

 lung. 



ORDER I. PECTINIBRANCHIATA. 



Shell spiral, external, operculated. Respiratory organ consisting 

 of a single pectiniform gill. 



intermedia, oblong, feebly arched below, moderately arched above, obliquely 

 truncated behind ; v. rostrata, oblong-oval, upper margin forming a dorsal 

 crest. 



11 v. complanata, oval, very compressed, brown, umbones close to anterior 

 margin ; v. radiata, marked with yellow and green rays ; v. ventricosa, very 

 tumid in the middle and umbonal region, larger, more solid, marked with green 

 and yellow rays ; v. minima, shell smaller, not quite so broad, blackish. (By 

 some conchologists Anodonta anatina is considered to be a variety of A. cygnea, 

 but a recent examination of over seven hundred specimens does not warrant 

 aie in doing likewise.) 



