26 THE NAUTILUS. 



entry average larger than those from the Eastern States. The 

 largest examined are from Lincoln, Neb., one of which is represented 

 by fig. 6, which has the apex more obtuse than others from the same 

 lot. A series submitted by Dr. Sterki, from Ohio, are uniformly of 

 medium size, none of them attaining the dimensions of specimens 

 from Michigan and further west. Fig. 9 represents another western 

 form, larger, proportionately higher and with the anterior slope more 

 convex than in the typical form, and which in the longitudinal out- 

 line approaches the western form of tardus, and not infrequently is 

 very difficult to differentiate from it, while fig. 13 represents an 

 extreme form, unusually high, with nearly straight lateral slopes and 

 a very acute apex. 



While as a rule it is not difficult to separate this species from 

 tardus, and on the whole the two species seem sufficiently distinct, 

 it is by no means always easy to decide in regard to individual cases 

 or even large series. This has been particularly true in regard to 

 the series from Ohio, received from Dr. Sterki, in which the two 

 forms seem to run together almost inextricably. As shown by the 

 figures given of each species, none of several distinctive characters 

 relied on are invariable, the posterior slope is frequently quite as 

 concave in tardas as in rivularis, while the convexity of the anterior 

 slope is not uncommonly quite as great in the one as in the other, 

 and neither the position of the apex nor the outline of the peritreme 

 is an absolutely invariable character. The most that can be said is, 

 that while typically the two forms are quite distinct, and in the main 

 can be readily distinguished, nevertheless the variation is so great 

 that, as is often the case with all the fresh-water groups, the sup- 

 posed specific characters fail to give an infallible basis for determi- 

 nation, and the decision in such cases must be the result of the indi- 

 vidual opinion of the observer based upon his general conception of 

 the species as applied to the particular specimens in question. 



Taking this species as a whole, it may be said to be distinguished 

 from tardus by larger size, more depressed form, more acute apex, 

 which is more inclined toward the right side, and nearer the centre 

 (longitudinal) of the shell, the posterior slope is proportionately 

 longer and more oblique, the anterior slope is usually not so convex, 

 the left slope is usually longer and more convex than the right, the 

 shell is usually decidedly wider anteriorly and the transverse section 

 is wider in proportion to its height, and the side lines consequently 



