BRACHIOPODA. 
259 
These features, which are shown in the figures herewith given, are accompanied 
by a correspondingly primitive expression of the exterior, the form of the shell 
being terebratuloid with the usual oblique cardinal slopes and regularly everted 
margins. 
I’lG. 178. Fig. 179. 
Looji of ItensselcBria mutabilis, Hall. 
Fig. 178. View from above; showing the perforated hinge-plate and thefsimple irregular line of coalescence of the 
lateral processes. 
Fig. 179. View from in front; showing the upward curvatui'e of the anterior plate and the median ridge on its under 
surface. (C.) 
Of the species which have been referred to Renssel^ria, R. Suessana, Hall, 
from the Oriskany fauna of Maryland, and also known in the lower Oriskany 
of the Hudson River valley, presents difference.s in form and structure suffi¬ 
cient to render its association with R. ovoides unnatural and unsatisfactory. 
These shells are lentiform in general contour; moderately and subequally con¬ 
vex ; 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 pseudarea, and the short inflection of the margin beginning here 
is continued along the lateral portion of the shell, where it meets a similar 
marginal inflexion from the opposite valve. These produce the sharp introver¬ 
sion of the lateral margins which is also one of the characteristics of the 
genus Megalanteris. 
The surface of the valves is covered with fine, hair-like radiating striae, 
which often are visible only near the margins or at their thickened extremi¬ 
ties on the inllexed portions of the shell. 
On the interior the dental lamellae are short and do not rest upon the valve. 
The hinge-plate is supported by two vertical septa, the median cleft and visce¬ 
ral foramen are more or less obscured and with sometimes a bilobed callus in 
