(I tHE NAUTILUS. 



margin slightly curved in a steep slope to the slightly angular, 

 rounded anterior end ; color light yellowish horn in the young and a 

 zone along the margins in older species, in which the upper parts 

 usually are grayish ; surface slightly shining, finely and irregularly 

 striated, with some deeper lines of growth usually of darker color; 

 shell rather strong, nacre whitish to grayish, muscle insertions 

 distinct; hinge stoutj strongly curved, plate moderately broad; 

 cardinal teeth short, the one in the right valve curved, its posterior 

 part thick, and usually grooved ; the anterior of the left valve short, 

 stout, triangular, abrupt, with a deep groove, posterior short, 

 oblique, curved ; lateral teeth short, stout, high, pointed, the outer 

 ones in the right valve quite small ; ligament short, moderately 



strong. 



Size : long. 3.8 alt. 3.8 diam. 2.8 mill. 



Habitat : Crystal Lake, Benzie Co., Mich., collected (over 600 

 specimens) by Dr. R. J. Kirkland ; also in Illinois, Iowa and 

 Kentucky. 



Typical specimens are easily distinguished from all other species 

 except an extreme form of P. compressuni Pr., from the same 

 place, having rounded beaks without ridges. Yet they are, quite 

 distinct. P. peraltnm is somewhat variable: in some specimens, 

 there are small but distinct projecting angles at the scutum, or scu- 

 tellum, or both. Others are less high, and the beaks are not so full 

 and prominent. 



A few specimens (dead valves) from Havana, 111., had been re- 

 ceived from the Illinois State Laboratory of Nat. Hist. (Mr. Kofoid), 

 in 1895; a few valves from Iowa City, la., were sent, in 1896, by 

 Mr. Jas. H. Ferriss, and a few good specimens from Bowling Green, 

 Ky., by Miss S. V. Price in 1899. While all these evidently were 

 of the same Pisidium, they seemed not sufficient for establishing a 

 new species upon them, but now proved identical with the Michigan 

 form, and are valuable in showing a wide geographical distribution 

 of our species. 



Pisidia are becoming an important factor of our molluscan fauna. 

 Owing to the efforts and the kindness of many conchologists in the 

 United States and Canada, the writer had chances to examine a 

 large number of specimens over two hundred thousand, during the 

 last five or six years, besides ten thousands of Sphaeria and Calycu- 



