PART II. LIMNACIN^. 337 



Helicina Lam. Heliciform ; aperture with an open 

 slit, or narrow fissure at the base. 



major Gray. Zool. Journ. i. sub-marginata. lb. pi. 6. f. 11. 

 68. pi. 6. f. 10. 



Pachytoma Sw. Aperture entire ; the inner lip very 

 thick ; the spiral whorls harcUy convex, 

 occidentalis. Zool. J. iii. 15. viridis. Zool. Journ. i. pi. 6. 

 f. 6—10. {fig. 28. p. 184.) f. 7. 



Oligyra Say. Aperture entire ; the base of the pillar 

 produced beyond, 

 rhodostoma. Zool. Journ. i, pi. 6. f. 9. orbiculata Sa^/. 



Trochatella Sw. Trochiform ; spire elevated, acute • 

 inner lip very thin ; outer lip spreading : represent- 

 ing Cyclophora. 



T. pulchella. Zool. Journ. i. pi. 6. f. 1. 



SUB-FAM. 5. LIMNACINiE, River Snails. 



Animal pulnioniferous ; tentacula two, often flat ; the 

 eyes basal {fig. 100. e) ; shell very thin; operculum 

 none. 



Plaxorbis. Shell discoid, both surfaces nearly alike ; 

 pillar none ; aperture oblique ; aperture reversed. 



Plano7'bis Drap. Spiral whorls few ; body- whorl ven- 

 tricose. 

 P. corneus. Drap. I. f. 42 — 44. Sow. Man. f. 311. 



Spirorbis Sw. Shell greatly depressed ; the whorls 

 very numerous. 



S. vulgaris. Drap. 2. f. 6, 7. 



Helisoma Sw. Shell ventricose, the spire sunk below 

 the body-whorl ; whorls hardly three. 

 H. bicarinata. Sow. Gen. f. 4. 



Segmentaria Flem.* Shell greatly depressed ; the 

 whorls very few ; the internal cavity divided, by 



* It is not a litile extraordinary, that no writer, that I am aware of, Bri- 

 tish or Continental, has adoj)ted this, the most remarkable tjpe among the 

 aquatic Pulmonaria. Dr. Fleming deserves much credit for being the first 

 who characterised it. 



