Foerste.] 
318 
[May 1, 
specimens no differences can be noted. The largest specimen is 
6.9 mm. long and 7.2 mm. broad. 
Rhynchonella scobina , Meek, from the Ohio Clinton is a much 
larger species, often attaining a length of 15 mm. and a width of 
* 18 mm. With the increase of size the mesial sinus and fold are 
much more prominent. The peculiar surface ornamentation, con- 
nected with a series of fine close concentric striae, which has given 
name to the species, will also serve to distinguish it. 
At Cumberland Gap, Tennessee, were found a number of valves 
in which the plications were well defined but showed no sharp dif- 
ferentiation into side and mesial lobes ; there is also no good mes- 
ial sinus. The largest specimen seen has a length of 6.2 mm. and 
about the same width. There are about thirteen plications on each 
valve. These specimens are here doubtfully referred to R. neglecta. 
Rhynchonella acinus, var. convexa, var. nov. 
(PLATE VI, FIG. 13.) 
The result of observations on that division of the genus Ortliis 
known as Platystropliia has been to place little value upon the 
number of striae on the mesial fold or in the sinus, except as vari- 
etal distinctions. Davidson in his studies on British brachiopods 
has arrived at similar conclusions in regard to the genus Rhyn- 
chonella. Thus under R. borealis , Schlotheim, are placed forms 
having from two to seven radiating striae or plications on the me- 
sial fold. Our own experience with American forms does not ad- 
mit of similar sweeping generalizations. Yet there is no doubt 
that the number of striae vary in those forms having many striae, 
and that in those forms which have the number of striae typically 
reduced to two on the fold, careful search will always result in 
finding at least a rudimentary third plication on some specimen. 
We do not find any great difficulty in separating these forms from 
those in which the number of striae is typically greater. 
Rhynchonella bidens , Hall, is found in the Clinton Group of New 
York. It is a small species, length and breadth about the same, 
with a subcircular outline, both valves strongly convex, beak closely 
incurved, the concentric striae very fine. About five plications are 
found on each side of the mesial fold. Rhynchonella bidentata , as 
identified in the Niagara group of New York, is also a small spe- 
cies. The general outline of the shell is however more triangu- 
lar, due to the beak of the ventral valve which is acute and not 
strongly incurved. The dorsal valve is more convex than the ven- 
