THE NAUTILUS. 11 



Habitat: Michigan, all over the state; lakes in Wisconsin. 



Var. minutum, n. Smaller, less elongated, of more regular shape, 

 almost globular when fully grown, usually of deeper color, surface 

 with very fine striation, polished, but almost always covered with a 

 greenish or blackish coating. Aroostook County, Me., very com- 

 mon in some waters, collected by Mr. Olof O. Nylander ; Moliawk, 

 N. Y., in the collection of the late Dr. James Lewis ; also some forms 

 from Michigan rather range with the variety, e. g., from Hess lake, 

 collected by Mr. L. H. Streng. This Pisidium has been known for 

 years. But owing to the fact that Pis. rotiindatum Pr., of almost the 

 same size, was not exactly known, it was thought better to defer 

 publication. It is one of the best characterized of our species, not 

 nearly related to or resembling any other Pisidium, except P. ferru- 

 gineiim Pr., 1 which is at once distinguished by the strong ridges on 

 the beaks. Pis. medianum is mail ly characterized by its beaks being 

 almost exactly in the middle of t ie mussel, its anterior part being 

 sometimes even smaller than the posterior. Hence its name. 



Specimens have been collected in deeper water, 24 meters, of Lake 

 Michigan, off New York point,, and also from the stomachs of white- 

 fish, sent by Mr. Bryant Walker. Among a lot from Blue Lake, 

 Michigan, collected by Dr. R.'J. Kirkland, there were many speci- 

 mens with that peculiar, perpendicular scar so often seen in Pisidia 

 and Sphaeria, but unusually deep. 



7. Pis. kirklattdi, n. sp. Mussel of medium size, somewhat oblique, 

 well inflated when mature, very little so in the young, high, rather 

 oval in outline; superior margin strongly, inferior moderately curved, 

 posterior slightly truncated, passing into the superior by an obtuse, 

 rounded angle, antero-superior slightly curved or almost straight, 

 sloping toward the rounded anterior end; scutum well, scutellum 

 slightly marked; beaks somewhat posterior, high and prominent in 

 the mature, low in the young mussel, with stout ridges, highest at the 

 posterior and slanting towards the anterior ends, slightly sinuous on 

 the outer sides ; surface with very coarse, rather regular striation, 

 dull, rugulose, straw colored in the young, light grayish in the adult 

 with a light zone along the margin ; shell rather thick, nacre almost 

 glossy, appearing bluish in old specimens, muscle insertions distinct; 

 hinge stout, hinge plate broad ; cardinal teeth of moderate size, rather 

 high up on the plate, the right one angular, its posterior part thick- 



1 That species has, so far, been seen only from New England and New York. 



