LAMELLIBRAHCHIATA. 531 



Palloooueha minnesotensls.] 



My conviction that Psiloconcha does not belong to the Modiolopsidce rests partly 

 upon the resemblance just noticed, but more especially upon a difference in the 

 composition of the shells. That some difference, whatever its nature, really existed 

 a comparison of the fossils will render obvious at once. Thus, I have collected 

 from the same bed of shale species of Modiolopsis, Adinomya, Orthodesma and 

 Psiloconcha. The shells of the first three genera were coated with either a black 

 or dark brown film, while those of the last matched the color of the shales or were 

 a few shades lighter. 



A single and not very typical species occurs in the Galena of Minnesota. At 

 least seven and probably eight species are found at various horizons in the Cin- 

 cinnati group. Two of these are figured on the preceding page and one was 

 described by me in 1879 as Orthodesma subovale (Jour. Cin. Soc. Nat. Hist., vol. 2, p. 

 82). The others I hope to describe in the next report of the Geological Survey of 

 Ohio. 



Psiloconcha minnesotensis, n. sp. 



PLATE XXXVI, FIGS. 31 and 32. 



Shell rather small, moderately elongate, highest posteriorly, the length about 

 twice the greatest hight. Hinge line straight, nearly as long as the entire shell; 

 anterior end rounded, much narrower than the posterior; basal margin gently and 

 uniformly convex; posterior end subtruneate above, slightly produced and strongly 

 rounded in the lower half. Valves rather strongly convex for the genus, the great- 

 est convexity in front of and above the middle. Beaks small, situated between one 

 fifth and one sixth of the length of the shell behind the anterior extremity. Um- 

 bonal ridge subangular in the rostal region and unusually prominent for the genus. 

 Anterior to the ridge the surface of the shell is gently convex. Cardinal slope 

 slightly concave, abrupt near the beaks. Surface marked with fine concentric lines 

 which are thrown into obscure folds in crossing the umbonal ridge. Internal char- 

 acters unknown. 



This species is doubtfully referred to Psiloconcha. It differs from all the other 

 species of the genus in its greater convexity, proportionally narrow anterior end, and 

 comparatively prominent umbonal ridge. I might have placed it with Sphenolium, 

 Miller, but the shell is not sufficiently ventricose, the umbones are too small, and 

 there is no lunule in front of the beaks, while a slight gap separates the edges of 

 the valves at the ends. 



Formation and locality.— Middle Galena, Pleasant Grove, Minnesota. 



