INTERDUNAL PONDS AND TAMARACK SWAMPS 177 



L. desidosa are small, .6 inch long or less. The former has a 

 short conic spire with an aperture produced below, the latter, 

 a long, tapering spire and the aperture not produced. As the 

 name indicates the whorls of Planorhis are in one plane, so the 



Fig. 195, — Nymphoi damsel Ry, I schnuraverticalls. Enlarged. After Needham 



shell is flat. The mouth of the shell flares bell-like in P. campanu- 

 latus. P. hirsutus and P. parvus are small, . 25 inch or less in diam- 

 eter. In hirsutus the shell is covered with short, bristly hairs. 



■A * .' A. * - 





Fig. 196. — Buttonbush, Cephalanthus occidenlalis , pushing out into pond 



In P. parvus both sides of the shell (really top and bottom) 

 are equally concave. A small crustacean, a bender (Hyalella 

 knickerbockeri) is abundant, especially in the spring. In general 

 appearance it is much like Gammarus fasciatus (Fig. 56), but 

 while Gammarus swims on its side constantly, Hyalella does 



