CILIARY MECHANISMS OF LAMELLIBRANCHS 681 
of material, as in Schizotherus and others. The result is, as 
shown in figure 5”, that large and small quantities of material 
alike are usually carried in a broken, zigzag course to the lateral 
portion of the oral groove, much as indicated by the long feathered 
arrow. Small masses in the lateral groove pass on toward the 
mouth, while larger quantities are invariably caught up out of 
it by cilia of the ventral margin, and conducted to the palp tip. 
Visceral mass. ‘The ventral portion of the visceral mass is 
shown in figure 50 (vms). The foot has disappeared—there are 
no remains of foot muscles—in mature individuals, though a 
small foot opening persists far forward in the fused mantle edge 
(fo). The erystalline style shows through the thin epidermis 
at the point of the mass. The walls of the upper portion of the 
mass are ciliated, and conduct material to its posterior side, 
where it is cast off into the mantle chamber in the usual manner. 
Mantle. In the same figure, the mantle is shown to present 
no unusual features. 
Pholadidea ovoidea Conrad 
Specimens were taken from burrows about four inches deep in 
extremely hard clay, between tide lines on Puget Sound. Ana- 
tomically the form is very similar to P. penita, but unimportant 
differences are shown in figure 53. The lateral parts of the 
palps are enormously enlarged, especially the dorsal margins, 
which are fused extensively with the mantle (m) in the case of the 
anterior, and with the visceral mass in the posterior palps. 
The visceral mass is relatively more extensive than in the pre- 
ceding species, and of a different form ventralward. The foot 
is entirely absent, the sexual mass entirely filling the foot-like 
projection of the visceral mass; but a minute foot pore (fo) 
opens through the mantle edge far forward. There is a small 
forwardly projecting fold on the left mantle wall under the 
incurrent siphon. Whatever it may be, it has no relation to the 
ciliary mechanism. 
The ciliation of organs is like that of P. penita except on the 
palps, where the general current is directly across the folds 
