50 AMERICAN JOURNAL 



5. Xolotrema appressa, Say. 



Plate 9, figures 11, 7. 



Shell depressed ; spire only slightly raised, suture distinct ; 

 whorls 5, a little convex, the last subangulate at the periphery, 

 and constricted behind the widely-reflected lip of the aperture ; 

 obliquely ribbed-striate above, smooth beneath ; aperture trilo- 

 bate, with a long oblique parietal tooth and two small lip teeth, 

 of which one is on the upper and the other on the lower part of 

 the lip ; umbilicus filled by a deposit of callus. Yellowish horn 

 color. 



Diam. 18 mill. 



Principally west of the Alleghany Mountains, from New 

 York to Georgia, and westwards to Alabama, Tennessee and 

 Illinois. Wilmington, N. C, and City Point, Va., (Thomas 

 Bland). 



Much smaller in size, and without the hispid surface of X, 

 paUiata ; it is also flatter. Sometimes the lip teeth are not well 

 developed. (Fig. 7.) 



TRIODOPSIS Rafinesque. 



This genus diff'ers from the foregoing in the following particu- 

 lars : — The umbilicus is open, the basal tooth of the tridentate 

 aperture is tubercular instead of lamelliform, and the shell is 

 smaller. 



The species are widely distributed, some of them being found 

 in all parts of the United States, and a few as far to the south- 

 wards as Central America. The species of Triodopsis, like 

 those of some other groups, in their distribution seem to mutually 

 replace one another, thus forming, in the opinion of the older 

 writers, geographical varieties rather than species. That several 

 of them had a common parentage is obvious ; but, inasmuch as 

 the departure of each from the common type, though slight, ap- 

 pears to be permanent, these forms must be regarded as true 

 species. 



1. Triodopsis tridentata, Say. 



Plate 9, figures 6, 13. 



Shell depressed, spire somewhat convex, suture moderately 

 impressed ; whorls 5 — 6, a little convex, obliquely ridged-striate ; 

 aperture trilobate, lip widely refle3ted, on the parietal wall is 



