no. 1704. A COLLHOTION OF SHELLS FROM PERU—DALL. 163 
nodules on the inside of the ventral points of the valves. They 
spring from a thick reinforcement of the hinge line, simulating a 
hinge plate, and they have nothing to do with the muscular system, 
but, as in the Pholads, are buried within the mass of the body and 
are probably of use in supporting the internal organs against the 
violent shocks resulting from its boring operations. From the pos- 
terior end of the “‘hinge plate” to a point on the margin of the valves 
corresponding to the angle between the postmedian and _ posterior 
areas, extends in the adult a broad septum in each valve, continuous 
on its inner edge with the margin of the valves and on the opposite 
edge free, with arecess behind it equal in depth to about half the whole 
width of the septum. The surfaces of these two plates form the 
third myophoric area above referred to and carry a relatively im- 
mense mass of muscular fibers uniting and holding closed the two 
valves and counteracting the action of the muscles massed on the 
exterior myophores. In other words, these muscles correspond to 
adductors of ordinary bivalves as regards their function, while the 
external muscles operate like a ligament. The nodules on the inside 
of the distal or ventral ends of the valves are of a rather unusual 
shape, subcylindrical and blunt at the opposed ends, rapidly cuneate 
at the proximal ends. The type, an adult shell, (Cat. No. 207695 
U.S.N.M.), measures dorsoventrally 20, in length 19, and transverse 
diameter 19 mm. ‘The soft parts, in alcohol, of this specimen were 
about 8 inches (20 cm.) in length. The pallets, set rather far back 
from the siphonal ends, measured about 45 mm. in length, of which 
25 mm. is smooth cylindrical stalk, the remainder being vane, of 
which the mass is set on the stalk inequilaterally, the segments being 
closely crowded with a serrate profile, and pretty well covered by a. 
thin brownish periostracum which passes over the segments on the 
back without interruption for the interspaces. The width of the 
vane is about 5 mm. near the base, gradually narrowing to a point at 
the tip. 
It is somewhat odd that, im comparing the shell of this species 
with that of other Teredinidz, the most similar shell found was not 
that of any Xylotrya, but a shell belonging to another genus, the 
Teredo norvegica of Spengler. From this the valves of X. dryas 
differ in having the posterior area axially longer, the postmedian 
shorter, the premedian wider, and the anterior about the same pro- 
portion. The styloid processes are longer; between the root of the 
process and the anterior end of the thickening I have for convenience 
called a ‘‘hinge plate,” there is a small but prominent denticle which 
I have not found in any other species. 
It is not improbable that this species may be confined to the man- 
eroves and not attack dry wood; if so this would account for the 
form not being reported before. 
