315 
[Foerste. 
beak of the dorsal valve extending a moderate distance beyond 
a line connecting the postero-lateral angles, and the beak of the 
ventral valve extending beyond the latter, and moderately curving 
over, but not covering the same. Dorsal valve convex, with a 
medium fold clearly defined up to the very beak by a groove on 
either side, the grooves becoming strongly marked anteriorly. 
Ventral valve convex, with a well marked median sinus, well de- 
fined on either side by one or two stronger striae, further defined 
by the fact that from these striae the lateral lobes slope abruptly 
to the lateral margins thus increasing the prominence of these lim- 
iting striae, as well as of the beak, of which they form the most 
prominent part. The surface of both valves is marked by radiating 
striae and also by concentric striae often almost obsolete. About 
six or seven radiating striae are found in a width of 4 mm., usually 
six and three-tenths striae. The sides of the sinus make with each 
other angles of about 27.° The largest specimen is 11.8 mm. long 
and 12.2 mm. broad. The specimens differ from the typical forms 
in their smaller size and the fact that fine radiating striae are ab- 
sent, only one or two coarse striae of growth being occasionally 
present. 
A broader form of this type was correctly identified by F. Boemer 
from the Silurian strata of western Tennessee. 
Atrypa nodostriata , Bull. Denison Univ., Vol. i, PI. xm, fig. 9, 
from Soldiers’ Home, near Dayton, Ohio, is a ventral valve of this 
species. 
A single specimen 8.5 mm. long and 7.5 mm. wide, was found 
at Collinsville, Alabama ; apparently the specimen is slightly con- 
tracted laterally. It resembles the smaller forms of Atrypa mar- 
ginalis from Brown’s Quarry, but is narrower as already stated. 
The mesial fold and sinus are defined in the same way but are both 
narrower, about two radiating striae occupying the fold and a faint 
indication of one the sinus. Concentric striae exist and are rather 
far apart. 
The specimen of Rhynchonella plicatula, Hall, figured in Pal. 
New York, Vol. 2, PI. 23, fig. 9, g , suggests in the possession of 
postero-lateral angles some form of Atrypa marginalise but the type 
specimen has not been seen. The species itself is undoubtedly a 
Rhynchonella. 
Atrypa marginalise var. imhricata , Sowerby, was described from 
Great Britain. It is readily distinguished from the species by the 
