THE NAUTILUS. 135 



Grant in the " Sixteenth (1887) Annual Report of the Geological 

 and Natural History Survey of Minnesota," cites it from "Vermilion 

 Lake and all over St. Louis and Lake Counties," and says: "it is 

 found clinging to rocky shores and reefs, and seems to seek places 

 where the water is quite rough." 



Hanham in his recent " List of the Land and Fresh-water Shells 

 of Manitoba," published in The Nautilus, quotes simply Dawson's 

 original citation of the Lake of the Woods. 



These references, with Jay's citation in his Catalogue, which pos- 

 sibly may be one of the original lot collected by Long's expedition, 

 are the only ones, which I have been able to find, that can with any 

 probability be referred to Say's species. 



Having before me four different lots, aggregating sixteen speci- 

 mens, of what is undoubtedly the genuine corpulentus of Say, I can 

 confidently affirm that the species is entirely distinct from P. trivol- 

 vis, and must be accorded specific rank. 



Say's description is very exact, and when read so as to apply to a 

 a sinistral species, as this undoubtedly is, as shown by the young 

 shell (fig. 7), leaves but little to be added. 



The characteristic features of the species are the high, narrow, bi- 

 carinate, rugosely striate whorls, with widely separated raised growth 

 lines and large expanded aperture, which is higher than wide. The 

 superior surface is either almost perfectly flat, or more or less con- 

 cave, sometimes deeply so, varying as the shell is coiled horizontally 

 or somewhat obliquely to the axis; the superior carina, until the last 

 half of the last whorl is reached, is almost a right angle, the sides of 

 the whorl being but little convex, with the greater convexity below 

 the middle; the body-whorl enlarges very rapidly during the last 

 half of its growth, and become more ventricose, and both carina? be- 

 come less prominent; the superior, however, retains its position and 

 sensibly modifies the shape of the aperture, while the lower one 

 from the rounding out of the base of the shell, becomes subobsolete 

 and does not affect the convexity of the lower part of the lip ; the 

 umbilicus is large and crateriform, the base of the shell until the 

 body-whorl begins to enlarge towards the mouth being flat, and slopes 

 sharply from the carina into the umbilicus, so that the lower carina, 

 during that period of growth, is much more acute than the superior 

 one; the whorls of the young shell are very narrow and high, and 

 owing to the rapid increase in height in proportion to width, the col- 



