BRaonropopa. 843 
fully one-half the depth of the combined valves. Thespondylinm 
is very narrow and deep; combined with the median septum the 
height of these plates equals fully two-thirds the depth of the 
valve. The anterior margins of these plates are doubly incurved, 
the most projecting points being at the base of the septum, and 
at its lime of union with the dental lamella. The median 
septum consists of two vertical lamelle, each continuous 
with one of the component plates of the spondylium. 
The spondylium was the seat of muscular attachment, and it 
bears a series of fine radiating lines along its median portion, and 
transverse or concentric lines over its lateral slopes; the 
former probably representing the scar of the adductor, and the 
Fia@. 451. Fig. 452, ‘ 
Fia. 453.—Pentamerus (Conchidium) Knighti. A transverse section in the umbonal region. 
s Septum of the pedicle-valve. p. Dental plates forming ventral spondylium. 
8’. Septa of the brachial valve. b. Crural plates resting on the septa. 
Fia. 454.— Conchidium laqueatum, Conrad. Transverse section, showing the deflection of the 
median septum, the deep, narrow spondylium of the pedicle-valve, and the septa of the 
brachial valve bearing inclined crural processes. 
latter the impressions of the diductor muscles. In the brachial 
valve the beak is obtuse and closely incurved into the deltidial 
cavity or spondylium of the opposite valve. The dental sockets 
are long and narrow, their inner margins being bordered by two 
broad, convergent crural plates, which extend toward the bottom 
of the valve, but do not reach it. These sloping plates are sup- 
ported by two vertical septa, with which they are united, not at 
their extremities, but obliquely, just within their free edges. 
At the anterior angles of these free edges, there are two long, 
straight or slightly curved, rod-like crural processes extending 
into the anterior cavity of the shell. Beneath the beak isa 
faintly developed, bilobate or multilobate cardinal process. The 
muscular scars lie on the surface of the valve between the two 
95 
