BRACHIOPODA. 
85 
In the pedicle-valve the beak is inconspicuous and incurved, usually con¬ 
cealing the foramen and deltidial plates; frequently, however, the former is 
exposed. Cardinal slopes not well defined in the typical group. The convexity 
of the valve is greatest in the umbonal region, the surface sloping evenly 
to the sides, and becoming depressed on the median line into a sinus, which is 
most conspicuous on the anterior margin. Beak of the brachial valve not promi¬ 
nent; a median fold corresponds in strength to the sinus of the opposite valve. 
In the interior of the pedicle-valve the deltidial plates are usually absent; 
the teeth are prominent, recurved at the tips, and supported by stout dental 
lamellse, which are not produced anteriorly about the muscular area. Between 
them lies a deep, transversely striated pedicle-cavity, and in front of this an 
ovate muscular scar extending about one-half the length of the valve and 
divided into flabellate diductors (which are frequently very indistinct) and nar¬ 
row, cordate adductors. The pallial region is covered with ovarian pittings 
and branching sinuses. 
In the brachial valve the dental sockets are broad and deep. The hinge- 
plate varies considerably in form; in the typical division of the genus it is 
subtriangular in outline, and supported by stout crural plates. The median 
portion is flat or concave, the lateral margins thickened and elevated. At the 
apex of the plate and just within the beak of the valve is a circular perforation 
(visceral foramen), which is continued beneath the plate into tlie cavity of the 
valve. The anterior margin of the plate is straight or slightly concave, occa¬ 
sionally trilobate, and the crura are attached at the extremities of the lateral 
ridges. Sometimes the outline of the hinge-plate is rendered subquadrate by 
the development of two post-lateral expansions. 
The brachidium consists of spiral cones lying base to base, with their apices 
directed laterally. The form of these cones varies with that of the internal 
cavity, but as a rule they are much compressed vertically, the posterior curva¬ 
ture being short and convex, while the anterior curve is long and sometimes 
depressed. The crura originate from the hinge-plate at a large angle, are long 
and convergent; the primary lamellse arising from their extremities, make 
an angular curve at their origin, thence, in the typical species, curving deeply 
