74 Report oF THE State GEOLOGIST. 
triangular, concave on the upper surface and divided into two 
lobes by a median groove. The crura take their origin from just 
within the anterior margins of the lobes thus formed. In some 
species the hinge-plate is more subquadrate in outline, the varia- 
tion being produced by the development of post-lateral expan; 
sions. This plate is supported by a median septum, which 
extends for somewhat more than one-third the length of the 
valve. The crura are short and straight, and the primary lamellz 
of the brachidium originate from them at an acute angle, and 
come into closest apposition at the anterior extremity of the 
median septum. In the mature individual, the spiral ribbon 
makes about fifteen volutions, the bases of the cones being sub- 
parallel to the longitudinal axis of the shell and their apices 
directed toward its lateral margins. In their general shape the 
Fias, 308, 309.—The jugum of Meristella Walcotti. 
cones conform to the character of the interior cavity, and in the 
less convex species (IZ. Wadlcotti, MW. lenta), they are appressed 
on the side of the flatter or brachial valve. The structure of the 
jugum is the same as described for the genus Merrisra, with this 
difference, however: the circular arms of the jugum curve first 
outward in the horizontal plane, then backward and abruptly 
downward to the inner edges of the primary lamelle; in their 
return the same curvature is reversed and they, therefore, meet 
the stem of the jugum in the horizontal plane, their point of 
union being: invariably above the point of coalescence of the 
lateral branches of the jugum. 
The muscular area is elongate-ovate, and extends for the entire 
length of the median septum; the four adductor scars are 
sometimes distinctly seen, the posterior pair being broader and 
embracing the posterior extremities of the anterior scars. 
26 
