18 THE NAUTILUS. 



subject to less variations than many of the other species. For this 

 reason I hesitate to refer to it the more elliptical forms from the 

 western States, which are usually referred to it, but which seem to 

 me rather referable to A. kirklandi, and until a larger amount of 

 material shall have demonstrated the identity of these shells with the 

 typical form, prefer to restrict the species to the author's type, " dis- 

 tinguished by its circular and flattened form and central inconspic- 

 uous apex." As thus limited, it is found in the Delaware river at 

 Easton, Pa., the Ohio river at Pittsburg and Edgeworth, Pa., the 

 Illinois river, the Tennessee river at Knoxville, Tenn., and the Hol- 

 ston river, Tenn. The specimens from the last locality are those 

 quoted without identification by Lewis in his paper " On the Shells 

 of the Holston River" (A. J. of C., VI., p. 222), and later referred 

 to "haldemanif" (Proc. P. A. N. S., 1872, p. 110). Haldeman's 

 description, though brief, is quite to the point, and leaves little to be 

 added. It may be said, however, that the apex is smooth, the sur- 

 face smooth or delicately shagreened with fine transverse ripples, 

 which in none of the specimens examined become sufficiently raised 

 or connected to be called ribs ; the anterior and left slopes are 

 slightly convex, the posterior and right nearly straight ; the left side 

 is usually more arcuate than the right and often decidedly so, the 

 general shape, however, even then remaining subcircular. There is 

 some little variation in height as shown by the figures, and, in the 

 more elevated examples, the shell is less circular, the anterior and 

 left slopes become more decidedly convex and the apex rather less 

 central, being, as it were, tipped backward by the more rapid growth 

 and greater convexity of the anterior portion of the shell. The 

 largest examples seen are from the Ohio river at Edgeworth, Pa., col- 

 lected by Mr. George H. Clapp. Those from the Holston and Ten- 

 nessee rivers are decidedly smaller, the example measured from the 

 Holston being exactly typical in size. 



Fig. 13. Length 7, width 5.5, alt. 2 mm. 



Fig. 16. Length 7.5, width 5.5, alt. 2.5 mm. 



Holston River. Length 5.5, width 4.5, alt. 2 mm. 



Tennessee River. Length 5, width 4, alt. 2. 



Explanation of Plate 1. 



All the figures are drawn on the same scale. The outline figures 

 are transverse sections through the apex or point of greatest altitude. 



