770 Report oF THE STaTE GEOLOGIST. 
Meristina, Hall. 1867. 
(Plate 33, figs. 1-7.) 
Synonym; Whitfieldia, Davidson, 1882. 
Shells biconvex, the greatest depths of the valves being sub- 
equal. General expression meristoid. The beak of the pedicle- 
valve is erect in youth, but so greatly incurved at maturity as to 
totally conceal the foramen and deltidium. Cardinal slopes 
narrow but distinct, forming prominent shoulders which may be 
traced nearly to the middle of the lateral margins. A low, often 
indistinct median ridge extends from the apex forward ; at about 
the middle of the shell it is divided by a faint groove, becoming 
broader toward the margin and continued into a subnasute 
extension. Lateral slopes scarcely depressed. 
The brachial valve also bears a low median ridge, which mani- 
fests itself most conspicuously over the anterior portion of the 
shell. 
On the interior of the pedicle-valve the teeth are conspicuous 
and are supported by thin plates, which extend to the bottom of 
the valve and are produced forward to form the lateral boundar- 
ies of the muscular area. Between the posterior portion of these 
plates lies the deep scar of the pedicle-muscle, which is separated 
from the elongate and radially striate diductor impression by a 
prominent callosity. 
In the brachial valve the hinge-plate is deeply divided in the 
middle by a narrow sulcus, the two lateral lobes being elevated, 
and supporting the crural bases. The plate is thickened on the 
under side and supported by a median septum, which extends for 
one-half the length of the valve. The crura are short and 
straight, and the primary lamellz of the spiral ribbon originate 
from them at a sharp angle, diverge laterally as they turn down- 
ward, passing over a portion of the secondary volutions, approach 
each other toward the middle of their length, nearly meeting at 
the anterior edge of the median septum, thence again diverging 
to their anterior recurvature. The secondary volutions do not 
follow precisely the curvature of the primary lamellz and the 
resultant cones at maturity havea gracefully undulated surface. 
The jugum consists of two lateral branches, broad at their origin, 
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