CILIARY MECHANISMS OF LAMELLIBRANCHS 629 
A definition. In most forms, the narrow path traversed by 
material on its way to the mouth is very long. For convenience, 
it has been necessary to distinguish three regions in this tract, 
which may be called the oral groove. The first, always very 
narrow, is that nearest the mouth. It lies between the bases of 
the palps, which are here without folds. This may be called the 
proximal oral groove (pg, fig. 23). The second is the extension 
of the groove upward on the side of the body, where palps are 
widened and united—the lateral oral groove (lg, fig. 1). The 
third is a groove in the mantle wall, close to, and parallel with, 
the anterior edge of the inner demibranch, found in forms in 
which the outer demibranch does not extend so far forward as the 
inner—the distal oral groove (dg, fig. 1). In some forms (Myti- 
limeria), the lateral portion of the groove is absent; in others 
(Mytilus) there is no distal portion, while in the majority, all 
three regions are found. 
I am greatly indebted to Dr. W. H. Dall for the identification 
of several of the forms studied, and to Dr. Fred Baker, of San 
Diego, whose hospitable door is aways open to visiting zoologists, 
and who identified my San Diego Bay forms, and made it pos- 
sible for me to study them. 
GENERAL ACCOUNT OF CILIATION OF THE BODY OF 
LAMELLIBRANCHS 
Schizotherus nuttallii Conrad, var. capax Gould 
The location and function of cilia tracts on the outer surfaces 
of the bodies of lamellibranchs, are much the same in all species. 
They perform two primary functions, namely, the carrying of 
Fig. 1 Schizotherus nuttallii, var. capax; view of the right side, valve and 
mantle being removed; aa, anterior adductor; ap, anterior palp; b, bay of mantle; 
dg, distal oral groove; dm, dorsal margin of palp; es, excurrent tube of siphon; 
f, foot; ig, inner demibranch of gill; 7s, incurrent tube of siphon; lg, lateral oral 
groove; og, outer demibranch of gill; pa, posterior adductor; pp, posterior palp; 
sm, siphon membrane; vf, vascular fold; vm, ventral margin of palp; w, wall of 
waste canal; we, waste canal; x, point on palp face where folds are parted. 
Fig. 2 Schizotherus nuttallii; ciliation of left mantle fold; rsm, retractor 
muscle of siphon; x, posterior end of foot opening of mantle, its folds being fused 
behind this point; other letters as in figure 1. 
