OF DENISON UNIVERSITY. 
87 
XL Orthis Daytonensis, sp. n. 
[Plate XIII, Figs. 13 a, b, c, d.; Figs. 20 a, b; and Fig. 21.) 
Dorsal valve wider than long, very convex, greatest convexity 
being just behind the middle, thence sloping almost equally on all 
sides, except toward the postero-lateral regions, which are somewhat 
compressed ; a faint mesial sinus towards the beak. Cardinal area 
narrow, foramen broadly triangular, cardinal process narrow, com- 
pressed laterally, and situated beyond a line connecting the lower 
edges of the cardinal area, or on the line itself; not filling the foramen. 
Ventral valve flattish, its greatest convexity one fourth the distance 
from the beak or even closer; thence sloping towards the postero- 
lateral extremities and the anterior margin, causing the anterior portion 
of the valve to be depressed, and leaving that portion of the shell 
extending from the beak to about the middle of the lateral margins 
elevated above the other portions of the valve. 
The interior of a ventral valve found will be sufficiently explained 
by Fig. 20 b, of Plate XIII. A single specimen with both valves 
united has been found presenting the characteristics of the valves just 
described, and furnishes my authority for uniting them under the same 
species. But as a curious matter of fact most of the ventral valves 
have been found at Allen’s Quarry, and all the dorsal valves at the 
Soldiers’ Home Quarries. The entire specimen is smaller in size than 
most of the single valves found. 
Surface marked by 60 to 90 fine, radiating striae ; the branching is 
frequent and by intercalations. Concentric striae distant from each 
other about the space between the radiating striae, or more, giving some- 
times the appearance of quadrangular punctae between the striae ; con- 
centric striae usually best preserved in the spaces between the radiating 
striae, often not very plain on the striae themselves. 
Length of a dorsal valve, 17 mm.; breadth, 21 mm.; convexity, 
5 to 6 mm. Length of a ventral valve, 21mm.; breadth, 26 mm.; 
convexity, 4 to 5 mm. Length of the only complete shell found, 18 
mm,; breadth 23 mm.; convexity, 8 mm,; the relative elevation of the 
beaks in this specimen may be understood by examining Fig. 21, of 
Plate XIII. 
Locality a 7 id position. Allen’s and Soldiers’ Home Quarries, Clinton 
Group. 
