656 JAMES L. KELLOGG 
stream of water. It may be reasonable to assume, also, that the 
siphon membrane functions here, as in Schizotherus, in throwing 
heavily laden water downward. Like many structures, the 
mantle folds respond locally by muscular contortions when- 
ever they are touched, the fold invariably bending outward 
to touch the mantle edge, and remaining until the stimulating 
substance has been forced from between them. 
The extreme ventral edge of the mantle has cilia tracts of its 
own, the adaptation, in the separation of which, at y, and their 
meeting at x, is not apparent. 
Mytilimeria nuttallir Conrad 
This interesting species, specimens of which were taken 
among the San Juan Islands of Puget Sound, possesses a very 
thin, white shell, covered by a brown cuticle, and is imbedded in 
masses of ascidians, from which it gains protection. Shells 
vary in outline, sometimes being much distorted by pressure of 
surrounding objects. The typical form is illustrated in figures 20 
and 21, the latter being a ventral view, showing fused mantle 
edges, with a foot opening (fo) and a curious small supplementary 
opening (so) near the base of the incurrent siphon. An ossicle, 
present under the hinge, is fastened to the ligament. The 
extremely small foot (fig. 22 f) has the remains of a groove on 
its ventral side near the tip, the organ probably having fune- 
tioned in fastening byssus threads in the young. The anatomy 
of the form, of which little has been known, is more or less fully 
shown in the following figures, and perhaps does not need fur- 
ther description. 
Gills. The outer demibranch (fig. 22) is narrow and lies dorsal 
to the inner without covering it. Filaments are not united by 
ciliated patches as in Mytilus, but are united by concrescence. 
Cilia currents are down to the margin of the demibranch, and 
are continued downward across the outer lamella of the inner 
demibranch to its margin. The movement of particles is also 
downward to the gill margin on the inner lamella of the inner 
demibranch. 
