BRAOHIOPODA. 849 
Rensselzria, Hall. 1859. 
(Plate 46, figs. 1-15.) 
Shells ovate or elongate-ovate in outline, subovoid in contour, 
broadest posteriorly ; valves unequally convex, with a more or 
less distinct median elevation on both. Cardinal slopes broadly 
flattened in the typical species. Lateral margins compressed, 
often strongly inflected. 
Pedicle-valve with full, scarcely salient umbo; beak acute, 
incurved, with terminal foramen. Beneath and on each side of 
the beak is a'‘concave, sharply defined space, but no proper cardi- 
nal area. Distinct deltidial plates divided by a median suture 
may be present, but they are usually concealed by incurvature or 
atrophied by the encroachment of the umbo of the opposite 
valve. On the interior the teeth are prominent butnot thickened, 
widely divergent and close within the margins of the valve. 
They are supported by stout dental plates which rest for most of 
their length upon the bottom of the valve, but are free along 
their posterior margins. These plates are closely appressed 
against the lateral walls of theshell and become coalescent there- 
within later growth-stages. The muscular impression is restricted 
to the posterior portion of the valve, covering a very narrow lon- 
gitudinal area. Between the dental plates at the bottom of the 
pedicle-cavity lies a narrow median scar, and in front of this, a 
very deep, elongate impression, which is sometimes clearly divided 
into central adductor scars embraced by the diductor impressions. 
Over the post-lateral slopes are numerous fine, irregularly rami- 
fying sinuses. 
The brachial valve is considerably the less convex and is often 
flattened. The beak is minute and usually obscured by the over- 
lapping pedicle-valve. The hinge plate is large, flat, triangular, 
sometimes thin, often thickened on its posterior portion and rest- 
ing on the bottom of the valve. It is separated from the lateral 
shell-walls by narrow dental grooves, widening at their extremi- 
ties. Normally this plate is perforated by a visceral foramen 
entering at the under side and opening at or beneath the apex of 
the beak, but usually this perforation is closed by adventitious 
deposits. The crura are continuations of the upper portions of 
the supporting septa; they are broadened just beyond their base 
of attachment, and from their upper angles are given off the 
107 101 
