1889.] 
325 
[Foerste. 
prominent, apparently at one time having curved over and more 
or less concealed that of the dorsal valve. 
The mesial septum of the ventral valve was very strong, extend- 
ing in the largest specimens not quite one-half the length of the 
shell. The two hinge teeth, whose continuation anteriorly forms 
the single median septum, enclose a triangular space marked in the 
casts by a triangular projection. 
The dorsal valve has two hinge teeth continuing anteriorly as 
two median septa, forming a very low angle with each other, and 
usually extending in maturer shells a third or slightly more the 
distance of the shell toward the anterior margin. 
These two median septa are usually the only parts distinctly 
preserved in the dorsal valves ; but a third scarcely distinct median 
septum is not infrequently found j this septum ought perhaps to 
be called merely a low mesial ridge or striation. It lies between 
the two median septa already described, beginning at about half 
their length and extending between and beyond them to slightly 
more than half the length of the shell. In shells or casts still bet- 
ter preserved two oblong muscular scars are seen, one on each side 
of this mesial striation, and just beyond the two more distinct me- 
dian septa. In the very best shells or casts the mesial striation is 
seen to divide at its tip, the divisions outlining the median anterior 
part of the muscular scars ; in the centre of the fork thus formed a 
faint mesial groove originates, extending to almost within a third 
of the distance of the length of the shell from the anterior margin. 
These characters can only be noticed in well preserved specimens 
and the two median septa first described are the only characters of 
the dorsal valve universally present. 
Leptocxelia hemispherica, Sowerby. 
(PLATE VI, FIGS. 18, 19.) 
Leptocoelia. hemispherica is described by Hall from the Clinton 
Group of New York. 
Atrypa jlabella, Shaler, from South-west Point, Anticosti, is the 
same species. 
At Collinsville, Alabama, the species is common. From Ring- 
gold, Georgia, the same form has been collected, and at the Cum- 
berland Gap localities it occurs in great abundance. In view of 
its great abundance and its great geographical range it seems 
strange, that it should not have been found either in Ohio or In- 
