THE NAUTILUS. 67 
flated ; beaks much posterior, rather large, prominent rounded ; 
superior margin short, little curved, or almost straight, scutum and 
scutellum well marked, forming projecting angles; the other mar- 
gins 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 grayish 
or brownish-horn in old specimens, and whitish on the beaks; nacre 
glassy, inner surface microscopically rugulose ; hinge fine, short, 
cardinal teeth lamellar, the one in the right valve moderately 
curved, its posterior end thicker; the inferior in the left valve 
curved, the superior little so or almost straight; lateral teeth very 
short, very abrupt, pointed, thin, little projecting into the cavity of 
the mussel ; ligament small. 
Long. 4:0, alt. 3:6, diam. 2°8 mill. 
Long. 3°3, alt. 2°8, diam. 2°4 mill. or less (deep water form). 
The center of its distribution is in the region of the Great 
Lakes, where it seems to be common, especially northward, in the 
great and small lakes and rivers. It has been dredged from deep 
water in different places: Pine Lake, 5-11 meters; Lake Michigan, 
off New York Point, 24 meters; also taken from the stomachs of 
white fish of Lake Michigan. These deep water forms, almost all 
dead shells, were first seen among materials sent by Mr. Bryant 
Walker, in 1894. Later, fresh specimens in lots from different 
places in Michigan were sent by Mr. Bryant Walker, L. H. Streng 
and Geo. T. Marston; from different waters of the Mississippi drain- 
age, in Minnesota, by Mr. H. E. Sargent. A few specimens, in two 
identical lots, in Br. Walker’s and Roper’s collections, from Shen- 
don, Montana, at an elevation of 9000 feet, have much resemblance 
with our species, yet differ in some points, and it will take more 
materials to ascertain whether they are identical or not. 
This is one of our most characteristic Pisidia, distinguished, beside 
its surface features, color and the configuration of the hinge, by its 
oblique shape and the much larger anterior part. This character it 
has in common with Pis. virginicum Gmel. and walkeri ; the former 
of these is out of the question; the latter species is much more elon- 
gated, its beaks are much smaller, the outline is more angular, and 
the surface dull, from microscopic lamellae, but even. 
Pis. scutellatum is somewhat variable: the largest specimens seen, 
from Orchard Lake, Mich., are 4°5 mill. long. Those from deep 
