530 THE PALEONTOLOGY OF MINNESOTA. 
LPsiloconeha. 
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 Wykoff, 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. 
Fig. 42. a. and b, the left side and a dorsal view of an excellent cast of the interior of Psiloconcha 
grandis, n. sp., from the upper beds of the Cincinnati group, at Waynesville, Ohio. c. a right valve 
retaining the shell, and d. the right side of an internal cast of Psiloconcha elliptica, 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 Modiolopside 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 gommonly 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. . 
