THE NAUTII.US. 83 



anterior slope only slightly ^oTiveXjSeefiiro be also referable to tliis 

 species. 



Distinguished primarily by its sniall size, tiiis species is well char- 

 acterized by the prominent, obtuse apex, the short, straight posterior 

 slope and long, convex anterior slope. It is very close in general 

 appearance to A- shimekii, but seems to differ persistently in liaving 

 the apex, which is less excentric in position, projecting decidedly 

 upwards and not depressed at the tip as in that species, the anterior 

 slopes not quite so convex, while the posterior slope is longer, more 

 oblique and nearly straight. The extreme lateral compression char- 

 acteristic of sJtimekii is not present in this species, the right slope 

 being nearly straight ; this difference is particularly marked in the 

 immature shells, which in shimekii are decidedly narrow and pro- 

 portionately higher tiian the mature shell, while in pumilus the pro- 

 portions are nearly the same. Sterki's types are immature examples, 

 to which his statement, that '• the sides are parallel," is quite appli- 

 cable, but in mature examples the lateral margins expand a little and 

 become more curved as shown in the figure, which is from one of the 

 largest specimens seen, the dimensions being : 



Length 2.75, breadth 1.7.T, alt. 1.0 m.m. 



EXPLANATION OF PLATES. 



All the figures on each plate are drawn on the same scale, but 

 tliose on Plate VI are somewhat more enlarged than those on Plate V. 



Plate Y. 

 Figs. 1-3, A. parallelm Hald. Little Lakes, N. Y. 

 Figs. 4-6, A.parallelus Hald. Pine River. Marquette Co., Mich. 

 Fijr*. 7-9, A. parallelus Hald. Caribou, Me. 

 Figs. l(l-12, A. elatior H;dd. Kentucky. 



Plate VI. 

 Figs. 1-3, A. filoyus Con. Coosa River, Ala. 

 Figs. 4-6, A. filosus Con. Coosa River, Ala. 

 Figs. 7-8, A.filnsus Con. Cahawba River. Ala. 

 Figs. 9-10, ji. haldemani Bgt. Holston River, Washington Co., 

 Va. 



Figs. 11-13, A. haldetmini Bgt. Doe River, Tenn. 

 Figs. 14-16, A. borealis Mse. St. John's River, Me. 

 Figs. 17-19, A. sliiineh'i Pils. Deadman's Run, Nei). 

 Figs. 20-22, A. piiinihis Sterki. Tuscawaras River, O. 



