822 THE PALEONTOLOGY OF MINNESOTA. 
(Patellide. 
concentric folds, without radial lines. Muscular scars unknown. Type, P. typica 
H. & W. 
Scenetua, Billings. Shell conical, rather high, apex subcentral, surface with 
distinct radial lines or ribs crossed by fine lines of growth. Muscular scars large, 
situated above the mid-hight, forming a complete circle in which the impression of 
each muscle may or may not be distinguishable. Type, S. reticulata Billings. 
Lepetopsis, Whitfield.* Shell patelliform, broadly oval, conical, low, with the 
apex subcentral. Muscular impression horseshoe-shaped, open in front, consisting 
of anirregular band. Surface with concentric lines of growth, occasionally perhaps 
also with a few radial lines. Type, DL. levettii White, St. Louis group. 
SrenotTHeca, (Salter) Hicks. Shell small, high, the whole curved so that the 
apex generally projects beyond the basal margin. Surface usually with strong 
transverse (concentric) folds and fine radiating lines. Type, S. cornucopia Salter. 
Besides the above groups the Paleozoic strata doubtless contain others equally 
distinct that for want of material cannot now be characterized satisfactorily. The 
two Wisconsin Calciferous species which Whitfield describes in “Geology of Wis- 
consin” (vol. iv), under the names Metoptoma recurva and retrorsa, are, as has already 
been suggested by Lindstrém and Koken, certainly not Metoptoma. Nor can we 
consider them as being much nearer either Tryblidium or Archinacella. The peculiar 
retral bending of their apices is so unusual that it may well be considered as 
indicating a new generic type. 
Another peculiar type is shown in Metoptoma alceste and M. orithyia, both 
described by Billings in “ Palzozoic Fossils ” (vol. i), the first from the Hudson River 
group of Anticosti and the other from the Calciferous of Canada East. These are 
relatively high conical shells, with a narrow vertical fold or blunt carina on one 
side running from the apex to the margin. The general form and the fold remind 
one of Hercynella, Kayser, founded upon Upper Silurian shells of Europe, and it is 
possible that they should be referred to that genus. Koken, however, refers Hercyn- 
ella to the Calyptreide near Capulus and Platyostoma, a position we can scarcely 
believe proper for the two species in question. 
Metoptoma angusta Billings, from the Quebec group, is another species of doubtful 
affinities. It is a large shell, with the apex “a little in advance of the middle, and 
apparently a little curved backwards,” and “the anterior side flattened.” The last 
feature suggests Metoptoma, but we doubt very much that it really belongs to that 
genus. M. anomala, of the same author and formation, cannot be a Metoptoma, but 
belong to this genus, Tn Bunope the pews an represeied bi Mctoptomay EtLga ae putamen nie Gen airs 
and in wanting the truncation and flattening of the posterior side which characterizes the species of Phillips’ genus. 
Although Metoptoma has been used very frequently by American authors, it is quite clear that nove of the species described 
by them really possess the essential features of that genus. 
