850 Report or THE State GEoLoaIsT. 
jugal processes, which are long, pointing upward and inward but 
not uniting. From the lower angles the descending arms take 
their origin, following the curves of the valve, diverging for a 
short distance, thence abruptly approaching, and uniting to form 
a broad, elongate, acutely triangular plate, which is not sup- 
ported by a median septum, or otherwise connected with the 
valve. From the center of the posterior margin of the plate 
arises a small rod-like process, which extends for a short distance 
upward toward the crura. 
The muscular area is less clearly delimited than that of the 
opposite valve, and its component scars are not often distinctly 
defined. It is, however, broader and longer than on the pedicle- 
valve, and is divided transversely into anterior and posterior 
adductor scars, the former being the larger, and their surface 
covered by branching lines radiating from a median longitudinal 
ridge. From the narrow and somewhat elevated posterior extrem- 
ities of this area extends a broad median sinus, on either side of 
which arises a stout vascular trunk bending backward and over 
the post-lateral slopes. Secondary branches are given off from 
both its margins. These vascular markings of the genital region 
are rarely well defined. 
Surface of the shell distinctly plicated, with sparse concentric 
wrinkles near the anterior margin. The radial lines may be fine 
or coarse, but no species is known in which the surface is entirely 
smooth. 
Shell-substance punctated beneath the epidermal layer. 
Type, Rensseleria ovoides, Eaton (sp.). Oriskany sandstone. 
Distribution. Lower Devonian. 
Subgenus Beachia, Hall. 1893. 
(Plate 51, figs. 1-7.) 
Shells lentiform in general contour; moderately and sub- 
equally convex; both valves with an obscure and undefined 
median fold. The beak of the pedicle-valve is prominent, never 
incurved sufficiently to conceal its deltidial plates and foramen. 
The cardinal margin beneath the beak is flattened into a well 
defined pseudo-area, and the short inflection of the margin 
beginning here is continued along the lateral portion of the 
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