38 



Fresh-water Biology 



{Continued from page 32) 



7 — Shell rather thin, globose, whorls convex; animal with foot not produced beyond 



snout Viviparus 



— Shell thick and solid, more elongate whorls slightly convex, foot large, much pro- 

 duced beyond snout Campeloma 



8 — Shell flat, discoidal; operculum round, multispiral aperture round. Valvatidae 1 



genus Valvata 



— Shell globose or elongated, length under 10 mm., spire short Amnicolidae — 9 



9 — Aperture oval, length over 7.5 mm Bythlnia 



— Length under 7.5 mm 10 



10 — Shell smooth, outer lip of aperture thin Amnicola 



— Shell more slender and elongated; outer lip of the aperture thickened. . Paludestrina 



11 — Large; length more than 20 mm 14 



— Smaller shells; under 20 mm. Sphaeriid.\e 12 



12 — Umbone central, shell equilateral 13 



— Umbone subterminal, shell inequilateral Pisidium 



13 — Shell thin, rounded, polished, umbones quite prominent Musculium 



— Shell thicker, striate, umbones not so prominent Sphaerium 



14 — Shell elongated, laterally compressed, rounded in front, almost lacking a posterior 

 ridge. Beak sculpture consisting of a few parallel ridges following the growth 



lines Margaritana* 



— Shell oval to elongate; surface smooth or feebly corrugated Unio* 



— Shell thin, back sculpture consisting of several more or less doubly-looped parallel 

 ridges, often slightly nodulous on the loops Anodonta* 



* These were formerly included in one family, the Unionidae, but it has since been split up into many sep- 

 arate families and sub-families. Three common forms are here figured. For detailed information on the Bivalves 

 see the Ward and Whipple "Freshwater Biology." 



A spongilla fly, 5J5>Ta. 



Figures 4 and 6 show the operculum in detail beside the shell; 9, 10, 11, show it in the aperture. 



