THK NAUTILUS. 17 



habitants of running water, and are usually found on stones, dead 

 shells, etc., in rivers and streams. The smaller, stronger and more 

 compact shell is no doubt the result of their environment. A few 

 species, however, of which A. parallelus is the most notable example 

 are to be found on plants or dead leaves in slow-flowing or stagnant 

 water. The distribution of the species seems to be more general 

 than that of the species of Lasvapex. 



I. Ancylus rivularis Say (1819). PI. I, figs. 1-10 and 13-15. 



This was the first species of American Ancylus to be described and 

 the meagerness of the original description is probably to be attributed 

 to this fact. But one dimension is given and nothing whatever is 

 said in regard to the contour of the shell. It was not until A. tardus 

 was differentiated in 1840, that any information was given on the 

 latter subject and then only by inference. The consequent doubt as 

 to what form was really intended to be covered by the description 

 was recognized at an early date. The characteristic difference in 

 the position of the apex and the shape of the shell was first pointed 

 out by Adams (Thomp. Hist, of Vt., p. 164, 1842), and his remarks 

 on the prevalent uncertainty in regard to Say's species are quoted 

 with approval by Haldeman. It was not until the latter in his mon- 

 ograph supplied an accurate description and a good figure, that the 

 species can really be said to have been established. I am inclined 

 to agree with Haldeman rather than Tryon in considering Gould's 

 rivularis (Invert, of Mass., p. 224), to be parallelus rather than Say's 

 species. 



Taking Haldeman's description as the typical expression of the 

 species, A. rivularis is a well-marked form, which typically is easily 

 differentiated from all other recognized species. It has a general 

 distribution through the Northern States from New England to 

 Nebraska and southerly to Virginia and through the Ohio and Mis- 

 sissippi Valleys southwest at least as far as New Mexico. I have 

 not seen it, however, from any of the southern Atlantic or the Gulf 

 States east of the Mississippi. It is subject to considerable variation, 

 however, in size and contour, and it is not always easy to differentiate 

 it from the western form of A. tardus. It is not likely to be mis- 

 taken for any of the other eastern American species, except perhaps 

 A. haldemani, which differs in the particulars stated under that 

 species. 



( To be continued.} 



