530 THE PALEONTOLOGY ,OF MINNESOTA. 



LPslloeoneha. 



of the ventral margin, more compressed dorsal regions, sharper umbonal ridge, 

 and somewhat different posterior outline. The central and posterior parts of the 

 shell also are less convex. 



Formation and locality. Middle Galena, near WykolT, Minnesota. 



Genus PSILOCONCHA, n. gen. 



Shell elongate subelliptical, compressed convex, gaping slightly at both ends; 

 inequilateral, with very small beaks, inconspicuous umbonal ridges and smooth or 

 concentrically lined surface. Mesial depression very shallow or wanting;, basal 

 outline convex. Shell very thin; hinge plate very narrow, edentulous. Ligament 

 internal, linear. Muscular impressions exceedingly shallow, rarely distinguishable. 

 Anterior adductor scar small, subcircular or ovate, situated in front of the beaks 

 and just within the hinge line. Posterior adductor about three times the size of 

 the anterior, occupying the greater part of the middle third of the space between 

 the beaks and the posterior extremity of the shell. Pallial line simple, more dis- 

 tinctly impressed in the posterior half of the shell than in the anterior. 



Type: Psiloconcha grandis Ulrich. 



.d 



Fit?. 42. a. and 6, the left side and a dorsal view of an excellent cast of the interior of Psiloconclut 

 grandis, n. sp , from the upper beds of the Cincinnati group, at Waynesville, Ohio. c. a ritfht valve 

 retaining the shell, and d. the right side of an internal cast of Psiloconcha elliptfca, n. sp., from the same 

 horizon at Clarksville, Ohio, and Richmond, Indiana. 



The systematic position of this genus is doubtful. That it does not belong 

 to the Modiolopsidce I am satisfied, but where else to place it seemed a question 

 whose solution it was deemed best to defer till we shall have learned a little 

 more about certain Devonian and Carboniferous shells. Species of Psiloconcha, 

 in their gaping ends and general expression, remind greatly of Carboniferous 

 shells that are commonly referred to the recent genus Solenomya, but I cannot 

 bring myself to believe that the short end of the Lower Silurian species is the 

 posterior, as would be the case if they were related to Solenomya. Indeed, it 

 appears to me far from established that this is true even of the Carboniferous forms 

 referred to. 



