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 

 lamellae, 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 

 (viscercal foriimen), which is continued beneath the plate into the 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 outliiie 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 lamellae arising from their extremities, make 

 an angular curve at their origin, thence, in the typical species, curving deeply 



