826 THE PALEONTOLOGY OF MINNESOTA. 
[Tryblidium modestum. 
usual, curve around in front to meet beneath the apex. (See fig. lc, p. 820.) 
Lindstrém probably overlooked these anterior pairs because he noticed a slightly 
elevated, transverse, medially disconnected band just in front of the sixth pair. But 
this band is really the posterior boundary of the scars of an umbonal pair* of 
muscles which we have shown to exist in a number, if not all, of the Paleozoic 
Patellide. : 
As already stated, we regard T. conicum Whitfield, as a Scenella, while Metoptoma 
simplex Billings, which Whitfleld places under Tryblidium, and T. ptliolum Whitfield 
probably belong to Archinacella. T.? acutum Whitfield, with its flattened area-like 
slope beneath the projecting apex, seems to us to indicate an undescribed generic 
type. 
TRYBLIDIUM MODESTUM, ”. sp. 
PLATE LXXXII, FIGS, 1 and 2. 
Shell small, somewhat acutely ovate in outline, the apex being pointed, scarcely 
incurved and projectly slightly beyond the narrowly rounded anterior margin of 
the aperture; the latter is scarcely, if at all, arched. Surface not well preserved, 
apparently marked by faint lines of growth. Internal characters unknown. Length 
10.3 mm.; width 8.5 mm.; hight of apex about 2 mm.; greatest hight (near center of 
length) about 2.8 mm. 
This species is founded upon a single imperfect shell. As the muscular scars 
have not been observed the generic reference is perhaps a little doubtful. Still, the 
form of the shell is such that we are fairly confident that it will prove to be a true 
Tryblidium. Of described species 7. erato Billings sp., from the Black River lime- 
stone of Canada, appears to be the nearest. On comparison with the description of 
that species (it has not yet been figured) it is evident that 7. modestum is much 
smaller, relatively wider posteriorly and not so convex. The fact that the shell is 
narrower in front than behind will distinguish it at once from all the species of 
Archinacella. Otherwise it looks very much like A. patelliformis. It also resembles 
Stenotheca unguiformis, but is wider and the beak is not so prominent in front, while 
the surface markings are quite different. 
Formation and locality.—Black River group, Ctenodonta bed, near Cannon Falls, Minnesota. 
Collection.—B. O. Ulrich. 
Genus HELCIONOPSIS, n. gen. 
For generic description see page 821. 
Of this genus we have at present only three species, the two about to be described 
and an Upper Silurian one from Gotland, which Lindstrém placed doubtfully 
*These umbonal scars are shown in fig. 37, pl. 1, of Lindstrim’s ‘“ Gastropoda and Pteropoda of Gotland.” 
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