Foerste.] 
308 
[May 1, 
the median ridge in the interior of the dorsal valve and separates 
the muscular scars. Radiating striae are fine and closely set. 
Ventral valve strongly convex, especially near the beak and as 
far as the middle of the valve. Both exterior and interior casts 
were found. Hinge teeth, shown by the impressions in the cast to 
have been sharp and strong, are near the cardinal margin ; thence 
they extend as much less elevated ridges forwards, continually 
curving towards each other so as almost to meet at the median line 
in one of the specimens. The enclosed space is ovate-circular and 
contained at one time the muscular impressions. 
The muscular impressions of neither valve are distinctly pre- 
served . 
The cardinal angles are more or less rounded. The radiating 
stride are fine and close, varying from seven to eleven in a width of 
2 mm. 
The average specimen is 10 mm. long and 10 mm. wide for ven- 
tral valves, although one specimen with dimensions of 11 mm. has 
been found. The dorsal valve is usually a tenth or a ninth shorter 
in length as compared with the width. 
At Collinsville, Ala., similar specimens with more or less convex 
dorsal valves are found. Seven and three-tenths radiating striae 
are found in a width of 2 mm. The largest specimen is 11.4 mm. 
long and 13.2 mm. wide. 
In the Niagara at Waldron, Ind., this species has also a more or 
less convex dorsal valve, with a pretty well marked median de- 
pression. The radiating striae are somewhat stronger than in the 
forms just described. 
Specimens from the Niagara of New York show the characteris- 
tic mesial depression of the dorsal valve. In many specimens the 
anterior part of this valve is concave. The dorsal valve has not 
been observed to be ever as convex as in the forms just described. 
The striae are fine. 
All of the American specimens which have come under our obser- 
vation differ from the typical European forms in having much finer 
radiating striae. Indeed, for this class of Orthidae, the striae of the 
typical European forms might be called coarse. 
Orthis calligramma, Dalman. 
(PLATE VI, FIGS. 4, 5.) 
Orthis calligramma , Dalman, is characterized by the anterior 
