402 THE CHICAGO ACADEMY OF SCIENCES. 



more or less yellowish-horn. Hinge moderately strong, of the 

 same type as in Pis. abditum; cardinal 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 be- 

 tween the two cardinal teeth; ligament comparatively strong." 

 (Sterki.) 



Length, 4.50; height, 3.70; breadth, 2.80 mill. 



"In outline our species has some resemblance with P. vir- 

 ginicum, 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 stria- 

 tion finer, and the beaks are much smaller." (Sterki.) 



Animal: Not known. 



Distribution: Northeastern Maine (Nylander); Kent 

 County, Utica, Grand Rapids, Mich.; Columbia, Pa.; Mohawk, 

 N. Y.; Clearwater and Mississippi rivers, Minn. (Sterki); Du- 

 Page River, 111. (Ferriss); Lilycash Creek, 111. (Handwerk); 

 Francis Creek, Joliet (Ferriss). 



Geological distribution: Pleistocene; Loess. 



Habitat: Found on a muddy bottom in company with 

 P. compressum and P. abditum. 



Remarks: This is a rare species and has been found only 

 in the western region. It may be looked for from both south- 

 ern and western regions. 



157. PLsidium scutellatum Sterki, pi. xxxi, fig. 14. 



Pisidiu-m scutellatum STERKI, The Nautilus, Vol. X, No. 6, p. 66, Oct., 



1896. 



Shell: "Mussel of medium size, rather high, oblique, 

 markedly protracted downward in its anterior part, well 

 rounded, rather strongly inflated; beaks much posterior, rather 

 large, prominent, rounded; superior margin short, little curved, 

 or almost straight, scutum and scutellum well marked, form- 

 ing projecting angles; the other margins well curved, or the 

 posterior very slightly truncated, anterior end well rounded, 

 or with a slight indication of an angle; surface polished, with 

 irregular striae and some coarse lines of growth; shell thin, 

 transparent, of a yellowish-horn to amber color, often gray- 

 ish or brownish-horn in older specimens, and whitish on the 

 beaks; nacre glassy, inner surface microscopically rugulose; 

 hinge fine, short, cardinal teeth lamellar, the one in the right 



