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 Planorbis are in one plane, so the 



\ 



FIG. 195. Nymph of damsel fly, Ischnura verticalis. 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. 



FIG. 196. Buttonbush, Cephalanthus occidentalis, 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 



