BracuioropA. 807 
narrow. The muscular area is flabellate and extends for nearly 
one-half the length of the shell; it is composed of two elongate- 
ovate adductor scars inclosed by broad and radially striated 
diductors. A conspicuous median septum begins in the umbonal 
region and extends to within a very short distance of the anterior 
edge of the valve. 
In the brachial valve the hinge-plate arises from the bottom of 
the shell with a vertical anterior face; but just above the plane 
of the margins of the valve it is reflected in a curve so abrupt 
that its upper face becomes horizontal. The anterior face is con- 
cave and quadrate in outline; the posterior face is subtriangular, 
flat or concave, and is frequently bilobed at its extremity. In 
profile the plate has a hook-shaped appearance, and its posterior 
extremity is elevated considerably above the beak of the valve, 
and when the valves are in articulation, extending quite to 
the bottom of the umbonal cavity of the pedicle-valve. The 
crural bases are situated on the vertical face of the plate, 
just at the point of recurvature. The crura are slender, 
straight, long and rod-like, having a length equal to fully one- 
fourth that of the shell. They are attached at their tips to 
the inner surfaces of the primary lamella. The primary lamelle 
of the spiral coils are greatly incurved and their apices close 
together ; their umbonal blades are very broad. The jugum 
originates at about one-fourth the length of the lamella, is 
inclined slightly backward, the lateral branches uniting directly 
in front of the apices of the lamelle, and forming a single straight 
stem, which is continued beyond the opposite edge of the coil and 
almost to the inner surface of the pedicle-valve. The spiral rib- 
bon makes from six to ten volutions, and the cones have their 
altitude in the transverse diameter of the shell. 
The muscular area is very narrow and elongate, the posterior 
adductor scars enveloping the extremities of the anterior 
adductors. They are divided into pairs by a median septum of 
the same extent as that of the pedicle-valve. Fine racemose vas- 
cular sinuses are sometimes retained over the pallial region of 
both valves. 
The external surface usually bears a low median sinus and fold 
on the pedicle-, and brachial valves respectively. The epidermal 
59 
