BRACHIOPODA. 
143 
the shell; it is composed of two elongate-ovate adductor scars enclosed 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 with a vertical anterior face 
from the bottom of the shell; 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 concave and quadrate in outline; the posterior face is 
subtriangular, flat or concave, and is frequently bilobed at its extremity. In 
profile fhe plate has a hook-shaped appearance; its posterior extremity being 
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 lamellae. The 
primary lamellae of the spiral coils are greatly incurved and their apices close 
together; their umbonal blades are very broad. The loop originates at about 
one-fourth the length of the lamellae, is inclined slightly backward, the 
lateral branches uniting directly in front of the apices of the lamellae, and 
forming a simple straight stem, which is continued beyond the opposite edge 
of the coil and almost to the inner surface of the pedicle-valve. The spiral 
ribbon 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 vascular 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 layer of the shell is 
usually, probably always, covered with numerous fine, short spinules; these. 
