12. — Somaiogyrus 



Length 5 to 7 mm. S. integer (Say) Binn. 



Length 4 mm. ^ S. depressus (Try.) Gill. 



13. — Amnicola 



Length less than 3 mm.; whorls 4. A. granum (Say) Hald. 



Length about 4 mm.; -width almost equals length; whorls about 4, 

 very convex. 



Aperture usually subangular at base. A. parva Lea. 



Aperture rounded at base. A. limosa (Say) Hald. 



(A somewhat larger form, with less distinct umbilicus and 

 whorls slightly transversely wrinkled, probably a variety of 

 this species, has been reported from Iowa as A. porata 

 (Say) Hald.) 

 Length at least 5 mm.; whorls 4 to 6. 



Yellowish brown, 5,5 mm. long; whorls 6. A. sayana Anth. 



Green; 5 mm. long; whorls 4 or 5. A cincinnaiiensis Anth. 



14:.—VaUaia 



Spire elevated, whorls shouldered; usually tricarinate. 



V. tricar inat a Say. 



Spire fiat; usually bicarinate. V. hicarinata Lea. 



(The tricarinate form is known as V. hicarijiata var. normali.s 

 Walk.) 



15. — Helicina occulta Say. 



16. — GundJacJiia meehiana Stimp. 



17. — Ancylus 



Apex nearly central; not oblique. 



Apex rather acute; sides of narrow shell straight and nearly 

 parallel. A. paraUelus Hald. 



Apex obtuse; shell broadly elliptical, flattened. 



A. diapJianus Hald. 



Apex oblique. 



Length ^bout 6 mm. ; on rocks, etc., in streams. 



A. rividaris Say. 



Length less than 4 mm. ; apex radially striate. 



Shell laterally compressed; apex very oblique. 



A. sliimeJcii Pils. 



Shell not compressed ; apex less oblique. A. pumihis Sterki. 



18. — Lymnaa 



a — Distinctly more than 20 mm. in length when mature. 



1 — Bodywhorl inflated, much wider than spire; usually more than 

 25 mm. in length. 

 * — Apex obtuse or acute; shell more gradually enlarging, heavy, 

 opaque, brown within the aperture; body whorl not shoul- 

 dered. L. megasoma Say. 

 **-Apex acuminate; shell more transparent, without brown, 

 opaque epidermis. 

 Body whorl not shouldered, white within the aperture. 



81 



