I.. \MKI.I. ll:i:.\N( MIA 



\h conviction tha' m7/., does not l.clon- to ti // resU partly 



upon the resemblance ju.-t noticed. l>ut more especially upon a difference in the 

 composition of the sheik That -ome difference, whatever it* nature, really existed 

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

 from the same bed of shale species of I/ . / .lopsis, Aetinmy+, Orthodetmn and 

 /'M'/.X-KW/I./ The -li.-ll- ..f tin- tiiM three genera were coated with either a black 

 or dark In-own film, while those of the last matched the color of the shales or were 

 a few shades lighter. 



A Millie ;md not very typi.-al species occurs in the Galena of Minnesota. At 

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

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

 rihed by me in 1^79 as Orthodesma subovale (Jour. Cin. Soc, Nat Hist., vol. _'. \>. 

 The other- I hope to describe in the next report of the Geological Survey of 



Ohio. 



P-II.OCONCHA MIHNK8OTKN8I8, n. Sp. 

 PLATB XXXVI. FI08. 31 and & 



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

 twice the greatest bight. 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 suhtruneate above, slightly produced and strongly 

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



onvexit y 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- 

 !"inal rid^e subangular in the rnstal region and unusually prominent for the genus. 

 Anterior to the ridge the surface of the shell is gently convex. <'anlinal slope 

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

 which are thrown into obscure folds in crossing the umhonal ridjre. Internal char 

 - unknown. 



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

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

 comparatively prominent umhonal ridge. I mi^'ht have placed it with tiphenolium, 

 Miller, but the shell is not -uliicienf ly \vntricose. the unbones are too mall. and 

 there is no luiinle iti front of the beaks, while a -iiu'ht pi|> separates the edges of 

 the valves at the ends. 



and locality. Middle Galena, Pleuant Orore, MlnneaoU. 



