Silurian Fossils 
11 
Pedicel valve a little more convex than the brachial valve, the 
I cardinal area of moderate height, forming an angle of about 30° 
I with the general plane of the brachial valve. Delthyrium wide, 
I the sides diverging at an angle of 65°. Muscular impressions small, 
the anterior margin about 6 mm. from the beak; the lateral 
margins converging anteriorly; outline reentrant in front of the 
linear adductor impressions, as in typical specimens of Orthis 
flabellites. 
Radiating striations angular, increasing by intercalations at vari- 
ous distances from the beak. About 13 to 15 striae originate at or 
very near to the beak; 28 striae originate at least within 3 mm. of 
the beak, so that about 13 to 15 striae must have been intercalated 
between the more primary striae within a short distance of the beak. 
Additional striae are added about 9 mm. from the beak, and along 
the margins of the shell a total of 60 striations may be counted. 
While the striae originate in a fasciculate manner they are not suf- 
ficiently different in size, and the primary striae are not sufficiently 
prominent to make the fasciculation at all conspicuous, differing 
in this respect from Orthis fasciata, Hall. Concentric striae, if 
present, were not noticed on the specimens at hand. 
Osgood bed : New Marion, Indiana. 
The shell is not considerably thickened beneath the muscular 
area of the pedicel valve, as in Orthostrophia strophomenoides, nor 
are the vascular markings conspicuous. The muscular area of 
the pedicel valve is not conspicuously smaller than in shells of this 
size belonging to typical Orthis. For the group of shells having 
the structure of Hebertella fissistriata, with numerous intercalated 
striae, with the brachial valve not exceeding the pedicel valve in 
convexity, but externally resembling Hebertella, the term Schizo- 
nema is suggested. This term should include apparently also 
Orthis fasciata, Hall, which is not a true Orthostrophia, and pos- 
sibly also Orthis fissiplica, Roemer. 
Hebertella (Schizonema) fasciata, Hall. 
{Plate IV, Fig. 71) 
Among the specimens found in the Osgood bed, at New Marion, 
in Indiana, is one which closely resembles the description given 
of Orthis fasciata from the Rochester bed of New York. The 
postero-lateral angles are broken off so that the extension of the 
