$28 THE PALEONTOLOGY OF MINNESOTA. 
[Archinacella. 
apex are a peculiar feature, though probably of the nature of “progressive tracts.” 
The obtuse angulation of the back of the shell distinguishes the species from 
all the Paleozoic forms of this family known to us. When not in a good state of 
preservation it might be confounded with Archinacella patelliformis Hall sp., but in 
that species the angulation of the back does not extend to the extremities of the 
shell, while the anterior end is wider, the apex higher and the surface without 
radial markings. 
Formation and locality.—Trenton group, Clitambonites bed, Goodhue county, Minnesota. 
Collection.—E. .O. Ulrich. 
Genus ARCHINACELLA, n. gen. 
Metoptoma (part.), BILLINGS, 1865, Palesozoic Fossils, vol. ii WHITFIELD, 1878, Geol. Wis., vol. iv. 
Tryblidium (part.), WHITEAVES, 1884, Paleozoic Fossils, vol. iii, p. 31. WHITFIELD, 1886 and 1889, 
Bull. Amer. Mus. Nat. Hist., vols. i and ii. 
For generic diagnosis see page 821.* 
The shells which we propose to refer to this genus are decidedly like those of 
Tryblidium, especially so far as the position of the apex and the surface markings 
are concerned. Their internal markings, however, are readily distinguished, that 
genus having the muscular scars in eight detached pairs, while they form a 
continuous band in Archinacella. Unfortunately the muscular imprints are in most 
cases very faint, even on well preserved casts, so that we are generally obliged to 
rely upon another character in determining the generic position. Namely, in all 
the species of which the muscular scars are known to occur in detached pairs the 
anterior outline is acuminate, or at any rate more narrowly rounded than the 
posterior margin. On the other hand, the anterior margin is as broadly rounded or 
wider than the posterior outline in all the forms of which it is known that their 
muscular scars are not detached. We have, therefore, considered it good practice 
to assume that when the anterior end is narrowly rounded the species is a Tryblidium 
and when this end is the wider the species belongs to Archinacella. 
‘There may be some doubt about the affinities of that group of shells in which 
the outline, as viewed from above, is almost circular or regularily elliptical. In no 
case have we been able to make out the muscular scars, although we have studied 
some well preserved casts. Still, as the form of these shells agrees best with 
Archinacella, and as we know nothing seriously opposing our view, we think it best 
to arrange them, at least provisionally, under this genus. Besides the species of 
* We omitted from the generic diagnosis one feature that ought perhaps to have been included, namely, a pair of scars 
(? museular) occurring one on each side of the apex. They lie on the outside of the usual muscular band and have been 
observed in two species, 4. powersi and A. (Tryblidium) canadensis Whiteaves. The latter is a Guelph species and, as shown 
in Whiteaves’ figures (Pal. Foss., vol. iii, pl. v), has these scars more strongly impressed (in the cast) and further forward 
than they arein A, powersi, 
