Sterki: North American Sph.eriid.e. 459 



7". Pisidium abditum Haldeman. 



Pisidiu?n abdilum Haldeman, Proc. Acad. Nat. Set. Philada., I, 1841, p. 53. 

 Pisidium abditum Prime, Mon. Corb., 1865, p. 6 (excluding some synonj-ms). 

 Cf. Sterki, Nautilus, XXVI, 1912, p. 6. 



\'ariable. A great many Pisidia have been named abditum, and 

 such misidentified and often mixed lots are the bane of collections, as 

 is also the case with Sphccriiim striatinum. 



It seems that P. abditum is not specifically distinct from the Euro- 

 pean fontinaJe Pfeiffer (not including casertanum Poli) also very vari- 

 able; at any rate they are plainly of common origin. Yet to sub- 

 stitute the older name at the present time would only add to the 

 already endless confusion. 



Habitat. — East of the Rocky Mountains; common in the Northeast; 

 apparentl}^ scarce west of Ohio; not seen from the Southwest and the 

 Gulf States. 



78. Pisidium abditum (?) lacteum var. nov. 



Mussel about 3.5 mm. long, somewhat elongate, higher in front of 

 the beaks than behind, and thus the beaks are apparently somewhat 

 inclined backward; anterior end more angular (rounded) than in P. 

 abditum; shell colorless, glassy-transparent to whitish, translucent to 

 opaque, usually with alternating zones of these shades. 



Habitat. — Massachusetts. Found at various places in the vicinity 

 of Danvers by the Rev. Mr. H. W. Winkley, and also collected by the 

 writer in brooks in the vicinity of South Framingham. The form is 

 represented in the Carnegie Museum by No. 7224 from Danvers. 



At this point it may be proper to call attention to the fact that P . 

 roscum- Scholtz (nee Jenyns) has been regarded as a distinct species 

 by some European authors, by others as a variety or subspecies of 

 P.fontinale Pfeififer. A North American Pisidium from New England 

 and eastern New York seems to be identical, and apparently connected 

 with P. abditum by intermediate forms. 



79. Pisidium subrotundum Sterki. 



Pisidium subrotundum Sterki, Nautilus, XX, 1906, p. 19. 



Near P. abditum, but is probably distinct. In over a hundred lots 

 in the collection every specimen is plainly P. subrotundum. More- 

 over the two forms have repeatedly been found associated, and always 

 easily separable. P. subrotundum is generally more northern and 

 appears to be more ancient. 



