GASTROPODA. 835 
Archinacella rugatina.] 
ARCHINACELLA RUGATINA, ”. sp. (Ulrich.) 
PLATE LXXXII1, FIGS. 5 and 6. 
This species agrees closely with A. richmondensis Ulrich, but may be dis- 
tinguished by its more distinct and slightly incurved apex, somewhat arched 
aperture and stronger surface markings. The latter are coarsely lamelliform in the 
outer third. 
Formation and locality.—Richmond group of the Cincinnati period, Middletown, Ohio. 
Collection.—H. O. Ulrich. 
ARCHINACELLA INSTABILIS Billings, var. INCURVA, . var. 
PLATE LXI, FIGS. 21—23. 
Metoptoma instabilis BILLINGS, 1865, Pal. Fossils, vol. i, p. 251. 
Original Description: “Shell small, depresed conical; apex acute, slightly 
incurved, situated over the anterior margin; aperture circular; surface finely striated 
parallel to the base. Width of an average specimen, 8 lines, height 4 lines.” 
A single imperfect specimen, from the Black River group of Minnesota, agrees 
so well with Billings’ description and figures of this species, that we hesitate to 
give it a distinct specific name. The specimen, it is true, is smaller than the New- 
foundland types and shows besides certain peculiarities that, if they could be proved 
to be constant, might justify a separation. The apex, for instance, is more incurved 
in the Minnesota specimen,—indeed it curves inward sufficiently to form a complete 
volution,—and the whole dorsal outline is more convex ina side view. Provisionally 
it may be designated as var. incurva, and it should be added to the list of Black 
River species mentioned by Billings on page 372 of his Paleeozoic Fossils, vol. i, that 
are represented by closely allied species in divisions I, K, L, and M of the Quebec 
group in Newfoundland. 
Formation and locality.—The types of the species are from division L, Quebec group, Table Head, 
Newfoundland. Var. incurva was collected by E. O. Ulrich near Cannon Falls, Minnesota, in the Cteno- 
donta bed of the Black River group. 
Collection.—H. O. Ulrich. 
ARCHINACELLA ROTUNDA, 1. Sp. 
PLATE LXI, FIGS. 24 and 25. 
Shell small, obliquely conical, moderately elevated, aperture circular, horizontal; 
apex situated almost directly over the anterior margin, apparently small, pointed 
and slightly incurved; in a side view the posterior part of the dorsal outline is very 
gently convex, but in nearing the apex the convexity becomes much stronger: 
