TilK NAUTILUS. 123 



as a whole ; the spire is depressed, not appearing above the superior 

 carina of the body whorl, the umbilicus is very wide and more shal- 

 low, exhibiting all the whorls. In tricarinata, on the other hand, 

 the whorls are more closely coiled, making a round, deep, funnel- 

 shaped umbilicus; the penultimate whorl is elevated and only the 

 apex of the spire is depressed. 



The relation between the two species is very similar to that be- 

 tween Pyramidula perspectiva Say, and striatella Anth. 



The recognition of the specific distinctness of these forms not only 

 involves some changes in nomenclature, but also renders necessary a 

 re-description of the various recognized forms. 



The following re-arrangement of the group is offered as an attempt 

 to satisfactorily differentiate the different forms. 



I. Valvata tricarinata Say (fig. 1). 



Shell turbinate, thin, translucent, shining ; horn-color or pale 

 green ; whorls about 3^ ; flattened between the carina? ; shouldered; 

 the upper surface sloping upwards from the carina to the suture ; 

 spire elevated, depressed at the apex ; lines of growth faintly 

 marked; suture distinct; body whorl tricarinate, superior carina re- 

 volving nearly to the apex, the peripheral carina on the whorls of 

 the spire being usually covered by the lower whorl; carina 1 sharp, 

 elevated and lighter in color than the body of the shell; aperture 

 circular, slightly modified, however, by the carinas; lip simple, 

 sharp, continuous, slightly appressed to the body whorl ; umbilicus 

 round, deep, funnel-shaped, bordered by the inferior carina. Height 

 3^, width 5^ mm. 



Cyclostoma tricarinata Say, Jour. Phil. Acad. Nat. Sci., I, 13 

 (1817). 



Canada and eastern United States to, at least, as far west as 

 Manitoba and Iowa, and south to Virginia and the Ohio River. 

 (Utah, Ingersoll.) As all gradations between the typical form and 

 the different varieties can be found in any considerable suite of 

 specimens ; it is deemed best to draw the lines between the recog- 

 nized varieties upon the presence or absence of well-developed 

 carina?. Angulations of the body whorl, however acute, should not 

 be considered in determining the position of any particular specimen. 



This species is one of the most abundant forms in the Post- 

 Pleistocene deposits of Michigan, and apparently was then more 



