THE NAUTILUS. 75 



names. Besides, they are not doubtful species, being well defined 

 and distributed over wide areas. Mr. E. W. Roper having examined 

 specimens wished the writer to publish these species. 



The descriptions will not be exhaustive, especially as to the 

 hinges, leaving that, as well as figures, for a monograph, but they 

 will be sufficient for recognition of the species. 



P. WALKERI n. sp. Mussel of medium size, elongated, rather in- 

 flated, resembling P. abditum, but is smaller than good sized exam- 

 ples of that species ; it is also more elongated, and the smaller and 

 moderately prominent beaks are more posterior ; thus the anterior 

 part is comparatively very long, with the end rounded, the posterior 

 quite short, truncated at the end; scutum and scutellum well 

 marked. The shell is very thin, translucent, the surface verv finely, 

 and sharply striated, often somewhat scaly, dullish, or with a silky 

 gloss; color grayish, along the margin more or less yellowish horn. 

 Hinge moderately strong, of the same type as in Pi*, abditum; car- 

 dinal tooth of the right valve moderately curved, its posterior part 

 thickened and with a deep furrow; below this tooth, there is a deep 

 groove formed by the inferior edge of the hinge list raised ; and the 

 same formation is in the left valve below the deep groove between the 

 two cardinal teeth ; ligament comparatively strong. 



Long. 4'5, alt. 3'7, diam. 2'S mill. 



In outline, our species has some resemblance with P. virginicum, 

 by its elongate shape, the beaks situated posteriorly, the long and 

 below somewhat sacciform anterior part. But it is much smaller, 

 its shell quite thin, the hinge finer, the striation finer and the beaks 

 are much smaller. 



P. walkeri is quite a characteristic and beautiful species which 

 cannot be mistaken for any other. It has a wide geographical dis- 

 tribution but seems to live not in great numbers together, as e. g. P. 

 abditum, compression and others do. For the first time I found it 

 in a lot of Pisidiu from Kent County, Michigan, in the collection of 

 Mr. Byrant Walker in whose honor it has been named , and in 

 another from Utica, Mich. Later it was seen from Grand Rapids, 

 Mich. (L. H. Streng), Columbia, Pa. (Bryant Walker), Mohawk, N. 

 Y. (in E. W. Roper's collection, from E. R. Mayo) ; Clearwater and 

 Mississippi rivers, Minn. (H. E. Sargent). 



P. POLITUM n. sp. Mussel of medium size, well inflated, rather 

 high, beaks slightly posterior, rather high and prominent, not full 

 but well rounded ; scutum and scutellum slightly marked. Supe- 



