THE NAUTILUS. 123 



size. It is extremely subject to erosion, and mature specimens with 

 a perfect apex are comparatively rare. If, as has been stated, milesii, 

 like exilis, is a sexual variation of subsolida, it is remarkable that it 

 has not been found associated with that species in localities where 

 that species is abundant, and it is still more remarkable that where 

 it is found, its slender form is persistent and equally characteristic of 

 both sexes. The only form of subsolida with which milesn can at all 

 be compared, is the slender form from the Mississippi Valley known 

 as 0. exilis Anth. (fig. 10). Just what the relations of this form 

 with the typical subsolida are, have never been satisfactorily ex- 

 plained. If, as generally considered, it is merely a sexual variation^ 

 it is a curious fact that it has never been found in any of the Michi- 

 gan rivers where the typical form is abundant. But however that 

 may be, while superficially resembling milesii in its slender elongated 

 form, it differs, like the typical subsolida, in the shape of the spire, 

 the less rounded whorls, and consequently less impressed suture, 

 shape of the lip and texture. It seems clear, therefore, that milesii 

 cannot be referred to subsolida even as a varietal form. 



There yet remains to be considered its relation to the congeneric 

 forms, with which it is frequently found associated. The character- 

 istic color and texture of C. rufa are always sufficient to distinguish 

 it, even when the erosion of the upper whorls has destroyed the out- 

 line of the more elongated milesii. 



Lewis was quite right when he grouped decisa and milesii together, 

 and it must be confessed that the exact relation of the two forms is 

 not free from doubt. Milesii is more closely related to decisa than to 

 any other species, and it is possible that when a greater abundance 

 of material can be had, it may be relegated to varietal rank. But 

 from our present knowledge, the forms seem quite as distinct as any 

 of the more closely-related species of Campeloma, which are recog- 

 nized, and it would seem better to keep them separate until their 

 specific identity can be unquestionably established. Compared with 

 decisa as it is commonly found, milesii is a thinner, more elongated 

 shell, with a more acute apex ; the upper whorls are more convex 

 and the suture rather more deeply impressed ; the aperture is smaller 

 and narrower. This difference is well shown in the two forms as 

 found together in the Pine River, Marquette county (figs. 9 and 12). 



Then, too, there is a marked difference in the shajjc of the young 

 when ready for extrusion. It will be remembered that Dr. Lewis 



