208 
GEOLOGY OF THE FOURTH DISTRICT. 
86 . 
8. Delthyris medialis. — View of upper valve and hinge area, with the deltoid aperture. 
8 a. Lower valve of the same, showing the broad expanding mesial sinus. 
8 b. View of the interior of the lower valve. 
9. and 9 a. Two views of a young shell of the same species?? 
Upper valve semicircular, much elevated in the middle ; lower valve very convex, beak 
prominent; surface marked by 32 to 40 rounded ribs, which are crossed by numerous laminae 
of growth. These lines of growth are slightly arched upon the rib, and there is frequently a 
depressed line extending along the centre of each, half way from base to beak. The area is 
large, curved, and striated transversely. 
I find this shell in the State Collection, labelled by Mr. Conrad, D. audacula; but it does 
not correspond with the fossil described by him under that name (Jour. Acad. Nat. Set. Vol. 
8, p. 262). This fossil is the most abundant of the Deltliyrides after D. mucronata. 
Localities —Moscow ; Seneca lake ; Pavilion ; Lake Erie. 
10. Delthyris fmibriata (Conrad, Jour. Acad. Nat. Sci. Vol. 8, p. 263). — Lower valve. 
10 a. Cast of the upper valve of the same. 
10 b. A small portion of the shell magnified. 
Upper valve sub-elliptical; lower valve nearly circular ; surface marked by about twelve 
rounded ribs, which are crossed by concentric fimbriated lamella}; beak very prominent. 
The shell of this fossil appears very fragile, and the cast is more commonly met with. 
Localities —Seneca lake ; Moscow ; York. 
