319 
[Foerste. 
tral. The concentric striae are neither fine nor coarse. Neither 
the mesial fold nor the sinus is strongly developed. The plica- 
tions are angular. Rliynclionella acinus , Hall, of Indiana, is also 
a small species. In its typical form it has an ovate quadrate 
outline with a sub-attenuate beak. The plications are thick and 
broad ; there are no concentric striae, or perhaps they are very 
faint. 
The dorsal valve is somewhat flattened along the middle and the 
ventral is more convex. The plications of the median fold do not 
usually differ conspicuously in size from the lateral plications, and 
may even be slightly narrower. The median sinus is also rather 
deeply impressed, and the plication within it is as narrow as, and of- 
ten narrower than the lateral plications. The most marked feat- 
ure is the strong lateral compression of the shell. In typical spec- 
imens this is emphasized by the fact that the anterior of the shell 
is abbreviated so as to give a subquadrate outline to the shell. In 
the type specimens figured by Hall, representing as a rule the 
more mature state, the anterior outline is more produced, the lat- 
eral compression is still marked, but the general outline of the shell 
is more ovate. 
In a third form from the same locality, representing perhaps 
the adult of this species, the form is subtriangular, or broadly 
ovate, and rather flattened, with no lateral compression. In this 
the two plications on the mesial fold are often broader than the 
lateral plications and the dorsal valve presents a rather flattened 
surface. 
The young of Rliynclionella Wliitii , Hall, also from Waldron, 
Indiana, are often difficult to distinguish from R. acinus. In typi- 
cal specimens, it is ovate triangular, there is no lateral compression, 
the shell is not obese, and the dorsal valve is convex, not flattened. 
In more adult specimens these features become more prominent. 
The plications are quite angular, and the two on the mesial fold 
are usually approximate. 
In large collections R. Wliitii and R. acinus are difficult to dis- 
tinguish. If my conclusions are correct R. acinus is distinguished 
by the flattening of the dorsal valve, not always well marked, and 
the low, broad plications which become indistinct toward the beak, 
and frequently obsolete at the beak. R. Wliitii has a more convex 
dorsal valve, the plications are angular, almost sharp, and are dis- 
tinct even at the beak. In both species the plications of the 
