832 THE PALEONTOLOGY OF MINNESOTA. 
[Archinacella valida, 
by regular, concentric, sublamelliform strie.* The present species differs further 
in wanting the obtuse carination of the dorsum and in having a smaller apex. 
Formation and locality —Black River group, Rhinidictya and Ctenodonta beds, Minneapolis, St. Paul, 
Cannon Falls. Chatfield, and near Fountain, Minnesota. Of all the patelliform shells occurring in Minne- 
sota rocks this is the only one that is reasonably abundant. We have seen about twenty specimens. 
Collections.—Geological and Natural History Survey of Minnesota; Geological Department, University 
of Minnesota; W. H. Scofield; E. O. Ulrich. ; 
Museum Register, Nos. 4067 and 8723. 
ARCHINACELLA VALIDA Sardeson sp. 
PLATE LXI, FIGS. 14 and 15. 
Tryblidium validum SARDESON, 1892, Bull. Minn. Acad. Nat. Sci., vol. iii, p. 337. 
Shell rather small, oblique-subconical, strongly convex, the dorsum narrowly 
rounded; outline elliptical, the width and length about as four is to five; aperture 
not arched; apex just within the anterior margin, laterally compressed, but little 
incurved in casts, the point about two-thirds of the hight of the shell above the 
edge of the aperture; beneath the apex the outline is deeply cut out in a side 
view. Surface markings consisting of rather obscure lines of growth. Length 
(small specimen) 16.5 mm.; width 13.4 mm.:; hight nearly 7 mm, In another 
specimen, proportionally larger, the length reaches 20 mm. 
We believe the specimens here figured and described are specifically the same 
as the one which Mr. Sardeson recently proposed to call Tryblidium validum. A 
comparison of our figures with his, it is true, brings out some differences, the 
outline of the aperture in ours being more regularly elliptical and the profile in 
the side view less convex centrally. It would appear, however, that in Mr. Sarde- 
son’s figure 2 the convexity is exaggerated, since in his description he gives the 
_ hight of the shell as equalling only half the width. And this is true of both the 
specimens here referred to his species. In this same figure 2 he shows three 
detached muscle scars, of so large a size that the whole ring would be made up of 
only three or possibly four pairs! But this would be so much out of the regular 
order that we are fully convinced that they rest on faulty observation. So far as 
our specimens are concerned nothing of the kind is to be observed. On the contrary 
we notice faint indications of the Archinacella band, and hence refer the species to 
this genus. 
Specifically A. valida is nearer A. (Carinaropsis) patelliformis Hall, from the 
Trenton of New York,than any other form known to us. The lateral profile especially 
is nearly or quite the same in the two species. The New York species, however, is 
smaller, has more distinct surface markings and is obtusely carinated on the back. 
* In comparing this and other species with A. patelliformis, we refer to the Trenton form only, and, as we have not seen 
the original types, we are obliged to depend solely on Hall’s figures and description for our conception of their characters. 
