THE NAUTILUS. 



Haldeman's descriptions (Mon., 1842), though short, when read 

 in connection with his description of rivularis y clearly differentiate 

 the form indicated. His figure was evidently drawn from the broad 

 form found in eastern Pennsylvania, and which justifies his remark 

 that tardus is proportionately broader than rimtlaris, when understood 

 as applying to the outline of the p*eritreme and not to the transverse 

 section. His dimensions, however, are quoted from Adams. 



Neither Binney, Tryon nor Clessin, have added anything to our 

 knowledge of this species. 



I have not been able to find a specimen that agrees either actually 

 or proportionately with the dimensions given by Adams, which indi- 

 cate a large, narrow and very high form. Western specimens of the 

 same length are uniformly wider and lower. The only Vermont 

 specimens seen are a pair in the collection of the Philadelphia Acad- 

 emy labeled " Vermont," and a small suite of immature examples 

 from Hartland (No. 58036) (pi. II., fig. 7). The former are quite 

 typical in their longitudinal and transverse outlines, but are very 

 wide, almost sub-circular in shape (4.5 x 3.5 and 4.25 x 3). The lat- 

 ter are similar, but more depressed. Similar specimens, interme- 

 diate in elevation (pi. II., fig. 4), have been found at Orono and in 

 the Aroostook River in Maine. In all these the posterior slope is 

 straight. An elevated form (pi. II., fig. 10) similar to the Vermont 

 shells occurs at Silver Spring and Columbia, Pa., and is very like 

 that figured by Haldeman ; the posterior slope, however, is more 

 oblique, and in some examples noticeably concave (pi. II., fig. 12). 

 All these shells, however, are alike in the comparatively small size 

 and great width, and should a larger suite show that this is the prev- 

 alent eastern form of the species it should probably be distinguished 

 varietally. The western, and no doubt the typical, form is quite 

 different. Larger, narrower and higher, it is typically a well-marked 

 form. The resemblance between Say's type (pi. II., fig. 1) and the 

 upper half of fig. 20, pi. II., is very striking, and I have no doubt 

 but that Say's specimen is an immature example of this common 

 western species, which is found abundantly from Ohio west to the 

 Mississippi Valley. It is, however, quite variable, as shown by the 

 figures, and, as already stated, at times very difficult to differentiate 

 from the equally variable forms of rivularis. The typical western 

 form is represented by figs. 17 and 20, pi. II., and is characterized 

 by its regularly oval outline, elevated obtuse apex, which is scarcely 



