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. virgin iettm Gmel. and walkeri ; the former 

 of these is out of the question; the latter species is much more elon- 

 gatedi 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 



