CILIARY MECHANISMS OF LAMELLIBRANCHS 667 
the visceral mass (not shown in the drawing) material is carried 
posteriorly, and cast off into the mantle chamber. The mantle 
ciliation is to the ventral edge and backward to the bay, b 
below the incurrent tube of the siphon. 
Figure 37 shows the anterior edge of the inner demibranch 
inserted far down into the lateral oral groove (lg) ,and united by 
concrescence along the center of its bottom. Streams from the 
outer demibranch come down the oral canal on both sides of 
this attached gill margin. The figure shows the abrupt turn of 
the lateral into the proximal part of the oral groove, which leads 
inward toward the mouth. There is a sharply defined current 
on the dorsal margin of each palp close to the upper ends of the 
folds, leading into the lateral oral groove. 
) 
Cardium corbis Martyn 
Specimens were examined at Orcas Island, in Puget Sound. 
This Cardium is found in shallow water, only partially buried 
in the sand, so that unusually strong tide currents or waves 
throw individuals out on the surface. By means of the powerful 
foot, burrowing is easily accomplished. 
Gills. The outer demibranch (fig. 38, 0g) is relatively small, 
and covers but a small portion of the inner (7g). Between the 
lines of their origin from the visceral mass, there is a considerable 
space (bounded by dotted lines in the figure). There is no groove 
on the rounded free edge of the outer demibranch, and col- 
lections on its outer surface are carried downward, around the 
margin, and upward on its inner face, to be taken by the broad 
ciliated tract on the wall of the visceral mass between the gills, 
to which reference has been made. Here they are carried for- 
ward, and then ventralward in the long distal portion of the oral 
groove, parallel to the anterior edge of the inner demibranch, 
then along the lateral part of the groove on its way toward the 
mouth. 
The inner demibranch (ig) is attached to the posterior palp, 
as shown in the figure, but this does not modify the course taken 
by collected material, which is the usual one—to the marginal 
