LYMN^ID^ OF NORTH AMERICA. 411 



the posterior angle of the shell; the so-called "Siphon" is capable of 

 great expansion, and when taking in air at the surface of the water 

 is sometimes extended to a length of nearly a third of an inch. Tht 

 pulmonary cavity fills about half of the body whorl, and is colorless 

 and transparent as seen through the shell, where the heart can also 

 be seen pulsating. 



Some animals are lighter than others, the extremes of light and 

 dark being very great. The animal in alcohol is almost jet black, and 

 the spots are sometimes scarcely visible. In a freshly-killed specimen 

 the liver is yellowish-white, with bright yellow patches here and there, 

 but in alcoholic specimens it turns brownish, as seen through the 

 transparent mantle ; the edge of the latter is greatly thickened and 

 fleshy. 



Jaw: (PI. VI, fig. J). Superior jaw, wide and low; dorsal mar- 

 gin broadly arched, smooth ; ventral margin with a narrow central 

 projection; anterior face of jaw striated. The lower lateral margins 

 are frequently produced into rounded prolongations. Color very dark 

 brown; lateral jaws similar to those of auricularia. 1 



Radula: (PL IX, figs. D, F). Formula : £_V|-f +¥+*+¥+!+« 

 (35-1-35) ; central tooth unicuspid, the cusp long and narrow, the cut- 

 ting point small and acute ; lateral teeth bicuspid, wide, the mesocone 

 wide, spade-shaped, the ectocone rather narrow; toward the intermed- 

 iate teeth the laterals become narrower; intermediate teeth tricuspid, 

 the entocone small and placed near the distal end of the mesocone; 

 the ectocone is small and is placed about half way up the margin of 

 the reflection ; there is a small denticle above the ectocone ; marginal 

 teeth serrated, the distal extremity four to six cuspid, the inner mar- 

 gin frequently with two small denticles. There are over 100 rows of 

 teeth. In a membrane from an Isle Royale specimen the second inter- 

 mediate tooth and the first two lateral teeth had the entocones broken 

 up into from three to five long, sharp serrations. (PI. IX, fig. F.) In 

 another specimen there were fourteen lateral teeth instead of the nor- 

 mal number, ten. 



Genitalia: (PI. XII, fig. G). Male organs: Penis-sac rather 

 long, cylindrical, 4.50 mill, long, 1.00 mill, wide; penis 2.00 mill, long, 

 or about half as long as penis-sac ; retractor muscle of penis 1.75 to 2.25 

 mill, long, very slender; retractor muscle of penis-sac 1.50 to 2.25 mill, 

 long, wide, band-like, enlarging as it approaches the penis-sac, to which 

 it is attached by numerous small filament-like muscles; the two re- 

 write statement made by the writer in his paper on the Gross Anatomy of 

 Limnsa, that only the superior jaw is present in this species, was an error; 

 no species of Lymnsea has been found without the characteristic lateral jaws. 



