THE NAUTILUS. 67 



Long. 7, alt. 6, diam. 4.7 mill. 



Long. 4.6, alt. 3.8, diam. 2.8 mill, (small, northern form). 



Habitat : Great Lake Region, Michigan to New York • also Min- 

 nesota, Illinois and Ohio (Ohio river drainage). It seems to prefer 

 quiet water, small lakes and slow-running rivers. 



Pisidium affine is related with nov-eboracense Pr. and with sargenti 

 St. (See the following sp.) From the former, it differs by the fol- 

 lowing characters : it averages larger, its beaks are larger, broader, 

 as especially noticeable in the young, the whole muscle is more full, 

 the hinge margin is less curved, the supero-anterior, as mentioned, 

 forms one long, unbroken curve, the anterior part is larger, the end 

 situated nearer the " base " and more rounded. P. nov-eboracense 

 retains its light, yellowish horn color, and the surface is rather shin- 

 ing, while older specimens of affine usually are light to dark grayish, 

 and the surface is more dull. 



Our species is somewhat variable. Specimens from Michigan, 

 especially Perch Lake, 1 Reed Lake, some other small lakes, and from 

 the Grand River at Grand Rapids are regarded as typical, and ex- 

 amples from the Little Lakes near Mohawk, N. Y., are rather the 

 same. In northern Michigan, e. g., in Mountain Lake, River Rouge, 

 Carp Lake, there is a smaller form, of darker color but typical shape, 

 collected by Mr. Bryant "Walker, and the same was found in Clear- 

 water Lake, Minnesota, by Mr. H. E. Sargent. A somewhat higher 

 form, with a slightly marked angle at the scutellum, is known from 

 Minnesota, Michigan, Buffalo, N. Y. (Miss E. J. Letson), and Meyer's 

 Lake, near Canton, Ohio (the writer). The latter has a straighter 

 striation and a somewhat waxy appearance of the surface. 



Whoever has carefully studied and compared a few suites of spe- 

 cimens at all stages of growth, will always recognize the present 

 species, as the young and half-grown are quite characteristic, while 

 some full-grown examples may present similarities with P. nov- 

 eboracense and sargenti. All these species are decidedly variable, 

 in several directions, and so it takes a good deal of material and 

 some experience in order to ascertain their claims for specific dis- 

 tinction, and to recognize aberrant and poorly developed forms. The 

 present one being a " critical " species, I trust the somewhat lengthy 

 expose on it will be excused. 



Pisidium sargenti n. sp. Mussel of medium size, somewhat oblique, 



1 Collected in large numbers by Dr. Kirkland. 



