HYDE: CAMAROPHORELLA. 41 
Brachial valve. — In the brachial valve there is a high median sep- 
tum, triangular in shape and extending half the length of the valve, on 
the apex of which the jugum rests after the manner of an inverted V. 
The septum supports a stout, very concave, triangular hinge plate 
which projects posteriorly into the pedicle beak considerably beyond 
the hinge line. The septum usually penetrates the plate as a pro- 
nounced ridge along the bottom of the concavity although in some 
individuals this is almost imperceptible (pi. '8, fig. 39-41; pi. 9, figs. 
44, 50). The plate is usually distinctly pointed posteriorly. The 
crura take their origin on the anterior margin of the hinge plate close 
to the lateral margins. The dental sockets are contiguous to the sides 
of the hinge plate; they are deep, narrow, and convergent posteriorly, 
the thickened edges of the spondylium sometimes even curving slightly 
toward each other beneath the crural bases so that the two valves 
completely interlock. 
Commencing in the beak under the hinge plate and extending 
anteriorly to the edge of the septum are two plates, one on each side, 
which taken together, constitute a platform for the attachment of the 
adductor muscles. Their proximal edges are implanted on the sep- 
tum at about half its height; the distal edges rest on the valve. Pos- 
teriorly the plates may rest for a short distance directly on the valve, 
but throughout most of their length each is convex toward the interior 
of the shell with a cavity beneath it. On each, between the outer 
margin and the middle of the plate, usually nearer the former, a pro- 
nounced longitudinal ridge is formed which may develop into a low 
septum (pi. 8, fig. 39). These ridges limit the muscular attachments 
on either side. Their position seems to be determined largely by the 
position of the primary lamellae of the spiral arms which swing down- 
ward close to the plates and just outside the lateral septa. Between 
the lateral and the median septa one or two indistinct ridges may be 
formed. The muscle scars are obliquely striate, the striae extending 
outward and posteriorly. Although the striae are occasionally dis- 
tinctly presei-ved, none that has been seen shows a division into ante- 
rior and posterior adductors. The writer's father found a specimen, 
however, which showed this division distinctly but unfortunately it 
was lost in transportation. No scars have been noted on the septum 
that might suggest that the muscles were attached in part to it. Some 
variation occurs. In one instance the plate on one side is placed con- 
siderably higher on the septum than is that of the other and this seems 
