118 THE NAUTILUS. 



NEW VARIETIES OF NORTH AMERICAN PISIDIA. 2. 



BY V. STERKI. 



( Continued.} 



Pisidium variabile Pr. var. brevius n. Much shorter than the 

 usual form, oblique, moderately to well inflated, beaks comparatively 

 large, generally slightly flattened on top ; color darker ; so far as 

 known, the mussels are usually smaller. This Pisidium is consider- 

 ably different from the " typical " form, and an analogue of P. 

 compressum var. confertum. 



Hab.: Michigan : Houghton Lake ; Gogebic Lake and Slate 

 River to same, Carp River and Lake, Ontonagon Co., Walker ; 

 Crystal Lake (Dr. Kirkland); Minnesota : Clear Water Lake (Sar- 

 gent); Keewatin: Attawapiskat River (Mclnnes, sent by Whiteaves). 



Var. hybridum n. Smaller than average variabile, of nearly the 

 same shape ; but the shape. of the beaks, surface appearance, color 

 and opacity of the shell are those of Pis. sequilaterale Pr. This is an 

 interesting form, and might be taken for a hybrid between the two 

 species, both of which were found at the same places ; but it is 

 markedly constant, so far as known. 



Vicinity of Mohawk, N. Y.: Wide Water Canal (Jas. Lewis col- 

 lection, now Mr. Bryant Walker's), Chepachet Pond (received from 

 Mr. H. E. Sargent). 



There are some other forms of Pis. variabile, rather characteristic 

 and constant, and with more material will probably prove real 

 varieties. Even the more typical form is rather variable with re- 

 spect to size, shape, striation, color, thickness of shell and hinge. 



Pisidium noveboracense Pr. The form regarded as typical seems 

 to be common in New York, Pennsylvania and Ohio, prevalent in 

 springs and small brooks. But the species is almost endlessly vari- 

 able and seems to readily respond, in that respect, to various kinds 

 of habitat, so that it often seems impossible to decide whether a 

 certain form has to be ranged under it or not. But there are a 

 number of forms characteristic and more or less constant, and must 

 be described as varieties. 



Var. expansum, n. Anterior part of the mussel higher ; supero- 

 anterior slope more curved, anterior end more rounded ; postero- 

 inferior curve generally broader ; outlines in general more rounded; 

 more regularly inflated, beaks less prominent, young specimens less 



