24 PROCEEDINGS OF THE ACADEMY OP 



breadth of the outer whorl from the dorsal to the ventral side, showing about 

 one-third of each inner volution. Whorls increasing rapidly iu size, particu- 

 larly at right angles to the plane of the shell, sloping on each side from near 

 the umbilicus (with a slightly convex outline) towards the dorsum, and round- 

 ing abruptly into the umbilicus on the ventral side ; eneh of those within deeply 

 embraced by the succeeding turn. Aperture transversely reniform or sublunate. 

 Surface ornamented with rather small, regular, rounded costse, which pass 

 nearly straight across the sides of the whorls, and arch slightly forward in 

 crossing over the dorsum. On the dorsal side, (where they are of uniform size,) 

 from thirty-six to forty of the costa3 may be counted to every turn. Each of 

 those commencing at the umbilicus is there usually a little enlarged, especially 

 on the larger whorls, so as to form a small, subnodose prominence. Beyoud 

 these they all (particularly on the inner whorls) bifurcate regularly once, near 

 the middle of each side, and on the larger turns others are also intercalated 

 between, so as to make the number on the dorsal side five or six times as 

 great as at the umbilicus. 



The septa are rather crowded and provided with variously branched and 

 deeply sinuous lobes and saddles. The dorsal lobe is about one-fourth longer 

 than wide, nearly obovate in form, and ornamented with three principal 

 branches on each side, the two terminal of which are larger than the others 

 and each provided on the outer side with two or three more or less digitate 

 lateral branchlets, while the inner parallel margins are merely sharply serrated. 

 The dorsal saddle is of about the same size as the dorsal lobe, a little oblique, 

 nearly oblong in form, and divided at the extremity into two tripartite and 

 obtusely digitate branches, of which the one on the dorsal side is larger than 

 the other ; below these it is provided on each side with two alternating lateral 

 branches with sinuous margins. The superior lateral lobe is narrower and 

 shorter than the dorsal lobe, and provided with two principal branches on 

 each side, the two terminal of which are much larger than the others, and of 

 unequal size, the one on the right or dorsal side being the larger. Both of 

 these terminal branches are distinctly bipartite, the subdivisions being orna- 

 mented with several branchlets and smaller digitations. The lateral saddle is 

 about half as wide and near two-thirds as long as the dorsal saddle, more or 

 less oblique and rather deeply divided at the extremity into two subequal, 

 bifurcating and obtusely digitate terminal branches. The inferior lateral lobe 

 is as long as the lateral saddle, but a little narrower, and ornamented with 

 three variously digitate terminal branches, the middle one of which is longer 

 than the others, a little oblique and not exactly central. The ventral lobe is 

 small, being less than half as long, and scarcely two-thirds as wide, as the 

 inferior lateral lobe, and provided with three nearly equal, spreading, digi- 

 tate, terminal branches. Between the ventral lobe and the umbilical margin 

 there are two small auxiliary lobes, the first of which has two or three digi- 

 tations on each side; while the second is nearly simple, or but slightly sinuous 

 on the margins. 



In the number and arrangement of the lobes and saddles of its septa, as well 

 as in their mode of branching, this species agrees very nearly with A. Ilalli, 

 (Meek and Hayden, Proceed. Acad. Nat. Sci. Phila., March, 1856, p. 70.) It 

 has, however, one more lateral branch on each side of i's dorsal lobe, and one 

 less on each side of its superior lateral lobe, than A. Ilalli; while all the di- 

 visions of its lobes and saddles are more spreading. Although so closely 

 allied in their internal characters, these two shells present marked differences 

 in form, as well as in their external markings, the species now under considera- 

 tion being much more ventricose and more coarsely ribbed than A. Ilalli. 



It agrees much more nearly in form with A. Barnrtoni, Meek, (Prof. Hinds' 

 Report, Assiniboine and Saskatchewan Expl. Expedition, pi. 11, figs. 1 and 2,) 

 from far up north, on Mackenzie's River ; but differs in having a smaller and 



[Feb. 



