852 Report or THE STATE GEOLOGIST. 
In the bottom of the rostral and umbonal cavity is a broad, 
usually ill-defined muscular area, from which radiates a series of 
vascular ridges and depressions extending into the marginal 
region of the valve. The diductor scars are situated posteriorly 
and deeply impressed ; between and in front of them is a narrow, 
elongate adductor scar which is rarely divided medially and 
often extends forward to, or beyond the center of the valve. On 
each side of the muscular impression is a thickened area, very 
narrow at its origin in the rostral region or pedicle-cavity and 
produced into divergent ridges, usually two on each side, and a 
fifth in the median axis. These may extend to the margins or 
disappear before reaching the middle of the valve and are 
variously subdivided by vascular grooves and sinuses emanating 
from them. 
In the brachial valve the hinge-plate is small, similar to that 
of Renssev#ria and AmpuicEnta in general form, but is of rela- 
tively less size than in the former genus and is not perforated by 
a visceral foramen opening beneath the apex. Two very 
narrow, almost linear and closely submarginal dental sockets 
extend nearly to the apex; within them le two broad, sub- 
triangular crural plates, which are divided by a triangular 
median fissure extending to the bottom of the valve. The inner 
anterior angles of these plates bear the slender crural processes, 
the extent of which is unknown. In mature individuals 
the apical portion of the hinge-plate is peculiarly constructed ; 
the latter areas become more or less completely united, without 
altogether obliterating the median triangular fis-ure, and above 
this point the surface is excavated into a spoon-shaped cavity, 
when the development is extreme, or is transversely angular in 
the average individual. At a short distance from the hinge-plate 
and in the bottom of the valve there arises a low median ridge, 
which continues for a short distance, separating the obovate, 
narrowly flabelliform scars of the anterior and posterior adductor 
muscles. The anterior scars are considerably the larger, and 
their surface is longitudinally striated. The vascular grooves 
and ridges are more obscurely developed than in the pedicle- 
valve. 
104 
