THE NATURAL HISTORY SURVEY. 257 



FAMILY 



Shell: Varying from long and pointed through oblong- 

 ovate and obtuse to discoidal; whorls numerous, varying from 

 planorboid to sharp pointed; aperture thin, lip simple, sharp. 



Animal: With a wide foot, rounded behind; velar area 

 preserved in the adult where they form two side lobes on the 

 head; tentacles rather long, triangular or filiform; eyes placed 

 on swellings at the inner bases of the tentacles; breathing 

 orifice on right side. Mantle prominent. Buccal apparatus 

 consisting of one median jaw and usually two or more lateral 

 accessory jaws. Radula with either a very small, simple cen- 

 tral tooth (Limn(za) or a larger bicuspid tooth (Planorbis), and 

 numerous lateral and marginal teeth of variable form, being at 

 first bi- or tricuspid, then tricuspid, and finally the reflection 

 lengthens and narrows, and becomes four or five cuspid, the 



Fig. 81. 



Mouth parts of LIMN.EA REFLEXA Say. (Original, from drowned 

 specimen.) A, superior jaw; B, lateral jaws; C, radula; D, lips. 



cusps being confined to the extreme distal part of the reflec- 

 tion, or serrating the outer edge (Planorbis). 



The genitalia may be thus described (for L. emarginata var. 

 mig heist): 



Male organs: (PI. xxxiii. Fig. E): As in the Limn&ida gen- 

 erally, the male and female organs are separate (with the ex- 

 ception of the hermaphrodite gland) and open by separate 

 orifices, that of the male being behind the right tentacle and 

 that of the female at the base of the neck near the pulmonary 

 opening. The penis sac (PS) is very large, of great length and 

 large diameter; it is wide at the exterior opening but narrow 

 at the end where the penis is attached. The penis (P) is half as 

 long as the penis sac, very long and slender, with a rounded 

 head, about two-and-a-half times the diameter of the neck of the 



