822 THE PALEONTOLOGY OF MINNESOTA. 



[Patellida;. 



concentric folds, without radial lines. Muscular scars unknown. Type, P. typica 

 H. &W. 



SCENELLA, 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 an irregular band. Surface with concentric lines of growth, occasionally perhaps 

 also with a few radial lines. Type, L. levettii White, St. Louis group. 



STENOTHECA, (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 Lindstrom 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 "Palaeozoic 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 Calyptrceidce 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 



* Bull. Aincr. Mus. Nat. Hist., vol. 1. no. 3. p. 67; 1882. All the known American Carboniferous patcllold shells seem to 

 belong to this genus. In Europe the genus Is represented by Metoptiima. Phillips, from which it differs in its rounded oiitllno 

 and In wanting the truncation mid flattening of the ponterior side which eh;u ;ictcn/.cs tin; species of Phillips' -emi-,. 

 Although Meloiiloma has heen used very frequently by American authors, it. is ,|uite clear that none of the species dcs.erlb.ed 

 by them really possess the essential features of that genus. 



