Foerste.] 
320 
[May 1, 
mesial fold may be approximate or divergent. R. bidens differs 
from R. Whitii by its greater convexity, and more circular outline. 
I have no means of determining the relationship of R. bidentata of 
New York with Swedish specimens of the same name. Hisinger 
says the Swedish form is smooth, and Dalman emphasizes that 
fact, saying that numerous specimens he had examined are about 
of the same size and are all without concentric striae, and the plica- 
tions are quite thick. The New York specimens are certainly sup- 
plied with sufficient concentric striae. The literature and speci- 
mens at my command are not sufficient to decide the matter. 
At Hanover, Indiana, a form is found in the Clinton group, 
which agrees with Rliynchonella acinus , Hall, in having broad, coarse 
plications, with no concentric striae or only very faint ones ; the 
plications become indistinct at the beak. The general form is 
ovate triangular or broadly ovate, much as in the third form of 
R. acinus described from Waldron, Indiana. Neither 3 r oung nor 
old specimens are laterally compressed to any very marked degree, 
although one or two approach the figured forms of R. acinus at 
Waldron. The youngest specimens are usually flattened horizon- 
tally, rather than obese, but soon become strongly convex, the 
convexity being especially marked in the dorsal valve, which in R. 
acinus is flattened. This increase of convexity is accompanied by 
a greater distinctness of the mesial fold and sinus towards the beak 
than in R . acinus of Waldron. The Clinton specimens may be con- 
veniently separated from the Niagara forms as Rliynchonella acinus 
var. convexa. 
Rhynchonella decemplicata, Sowerby. 
(PLATE VI, FIGS. 23, 24.) 
Specimens of the following description are found at Cumberland 
Gap, Tennessee. 
Dorsal valve with the two median plications becoming more el- 
evated and wider anterior to the centre of the shell, forming a dis- 
tinct mesial- lobe of moderate breadth. 
Ventral valve having a corresponding depression or sinus along 
its median line, of the same width anteriorly as the mesial lobe of 
the dorsal shell ; along its median line in the sinus a single plica- 
tion of ordinary size is found. 
Lateral lobes of both valves with five or six plications. Plica- 
tions acute but rounded along their summit. The convexity of the 
