778 Report or THE State Gxroxoaist. 
and just within the beak of the valve isa circular perforation 
(visceral foramen), which is continued beneath the plate into the 
cavity of the valve. The anterior margin of the plate is straight 
or slightly concave, occasionally 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 develop- 
ment 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 com- 
pressed vertically, the posterior curvature being short and convex, 
Fie. 311. 
Diagram of the structure of the brachidium in ArHyRis. 
ax. Spiral coil. s. Saddle. 
a. Primary lamellz of spiral coil. m. Fimbriated extensions of saddle. 
a’, Seeondary Jamelle, t. Stem of jugum. 
a’’, Umbonal blades. f. Arms of jugum, 
ec Crura. b. Accessory lamella. : 
l, Jugum, n. Fimbriz on outer margins of lamelle. 
e. Lateral branches of jugum, 
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 lamellz beginning at their extrem- 
ities, making an angular curve at their origin, thence, in the 
typical species, curving deeply upward and backward, to form 
the first volution. The spirals are connected by a jugum, which 
takes its origin on the first half of the primary lamellz, the two 
lateral lamellae converging, and uniting at about half the dis- 
tance across the base of the cones, to form a broad saddle with 
a convex upper surface; the anterior extremity of this saddle 
may be simple or divided; its posterior portion is narrowed, 
30 
