Silurian Fossils 
79 
Hebertella (Schizonema) fissiplica, Roemer. 
{Plate III, Fig. 54.) 
Shell plano-convex. Brachial valve nearly flat or slightly convex 
with a distinct but shallow median depression similar to that of 
Dalmanella jugosa; cardinal area very narrow, deviating but 
slightly from the general plane of the valve; cardinal process in 
form of a thin, simple, vertical plate, as in Orthis jlabellites. A 
thickened elevated median ridge extends from the deltidial cavity 
forward to a point a short distance beyond the center of the valve. 
Muscular impressions indistinct. 
Pedicel valve convex; cardinal area high and flat, but slightly 
if at all incurved at the beak, forming an angle of about 110° with 
the general plane of the brachial valve; delthyrium narrow as in 
Hebertella nists. In the specimens from Dixon spring, the shell 
is but slightly curved along the median line, from the beak to the 
anterior margin. In the larger specimens from Clifton, the curva- 
ture corresponds more nearly to that of Hebertella nisis. In gen- 
eral the shell slopes strongly from the beak to the lateral and 
anterior margins. Muscular impressions small and rounded, ele- 
vated at the margin slightly above the interior of the valve; margin 
slightly incurved anterior to the adductor muscle impressions; the 
latter are linear and occupy about one-fifth of the entire width of 
the muscular impressions. Size of muscular area small, corre- 
sponding to that of Orthis rather than that of Hebertella. Interior 
of valves radiately grooved along the border as in typical species 
of Orthis. 
Radiating striae about 22 within 4 mm. of the beak, increasing 
to about 45 at the margin, about 5 to 7 striae occupying a width of 
5 mm. At their origin the newer striae are much less conspicuous 
than the older striae, usually originating near the latter although 
sometimes inserted near the middle of the spaces between the older 
striae. The older strise are usually more conspicuous, resulting 
in an alternation of larger and smaller striae or in a more or less 
fasciculate arrangement. Radiating striae crossed by numerous 
fine, sharp concentric lines, usually well preserved between the 
strise. 
In fig. 5a, plate 5, of Roemer’s monograph on the Silurian 
Fauna of Western Tennessee, the posterior margin along the hinge 
area is drawn too concave to the right and left of the beak. In fig. 
