836 THE PALEONTOLOGY OF MINNESOTA. 



[Pa 



Beneath the apex the outline is decidedly concave. Surface of cast smooth. Muscular 

 imprint, distinct, linear, the posterior part of the loop bending forward a little in 

 the middle. Diameter about 7.7 mm.; bight 4.5 mm. 



The specimen upon which this species is founded is important because it 

 preserves the muscular imprint and shows that this agrees, in what we must 

 for the present consider essential features, with the imprint found in the typical 

 species of Archinacella. And it is upon the strength of this evidence that we refer 

 all the rounded Lower Silurian patelliform shells having the apex submarginal 

 to this genus. 



Compared with related species, A. instabilis, A. simplex and A. estella of Billings 

 are all higher. A. pileolum Whitfield sp. also is very similar, but as it belongs to a 

 much lower horizon (Calciferous) it is fair to assume that it is distinct. 



Formation and locality. Cincinnati period, Utica horizon, near Graf, Iowa, where it was associated 

 with Orthoceraa sociale Hall. 



Collection. E. O. Ulrich. 



Genus PAL^ACM^A, Hall and Whitfield. 



Palasacmeea HALL and WHITFIELD, 1873, 23rd Rep. N. Y. Mus. Nat. Hist., p. 242. LINDSTROM, 

 1884, Gastropoda and Pteropoda of Gotland, p. 58. 



For generic characters see page 821. 



We propose to use this genus for all those lower Paleozoic patelliform shells 

 having the apex subcentral and the surface marked in a concentric manner only. 

 In the typical forms, all of which are confined to the Potsdam and Calciferous 

 formations, the markings consist of wide (2 mm. or more) concave undulations. 

 This is true of P. typica H. and W., from the Potsdam of New York, P. irvingi 

 Whitfield, from the same formation in Wisconsin, and P. (Metoptoma) quebecensis 

 and P. (Metoptoma) orphyne of Billings, from the Quebec group of Canada. In the 

 lower Trenton species about to be described these undulations are an unsteady 

 feature and scarcely distinguishable in casts, while in the Gotland species (Upper 

 Silurian) they are represented by narrow ridges, separated by usually short flat, 

 rather than concave, interspaces on which very fine concentric lines are dis- 

 tinguishable. 



Little or nothing is known of the muscular scars of all these species except the 

 last, P. solarium Lindstrom. In this a "wreath of muscular impressions, nearly 

 coherent," occurs near the top of the conical cast. All that can be made out of the 

 scars of P. humilis appears to agree with Lindstrom's observations on the Gotland 

 species, and, as the former is featured just as we might expect an intermediate stage 

 between P. solarium and the Potsdam species to be, we may provisionally assume 

 that the scars are essentially the same in the latter as well. 



