CILIARY MECHANISMS OF LAMELLIBRANCHS 659 
Lyonsia (Agriodesma) saxicola Baird 
This is a North Pacific species, the specimen examined being 
taken in Puget Sound. There is an ossicle beneath the hinge; 
a thick epidermis over the shell, extending far beyond the shell 
ridge posteriorly; and a supplementary opening, posteriorly, 
through the fused mantle edges, characters found also in Mytili- 
meria. There is a groove for byssus secretion on the under 
surface of the foot, and the species is hermaphroditic. 
Comparing figure 27 with figure 22, and figure 28 with figure 
26, it will be seen that the ciliation of all surfacesin Lyonsia 
saxicola is like that of Mytilimeria, and one would thus presume 
that they are closely allied forms. 
Lyonsia californica Conrad 
This form was difficult to study on account of its small size, 
the shell being less than an inch in length. The ciliation, how- 
ever, was clearly essentially like that of the preceding species. 
Modiolaria nigra Gray 
One specimen was taken from Puget Sound. The shell was 
covered by a heavy pale green cuticle. This form presents some 
interesting anatomical features. As shown in figures 29 and 30, 
the mantle edges are fused anteriorly, but the ventral sideof the 
incurrent siphon is open, and communicates with the foot open- 
ing (fo). Muscular contraction normally brings the mantle 
edges together so as to separate these openings; but when water 
is heavily laden with particles, they form one large opening 
through which water enters the mantle chamber, as shown in 
Fig. 29 Modiolaria nigra. 
Fig. 30 Modiolaria nigra; mantle edge exposed by parting of valves; bg, 
opening of byssus gland; es, excurrent tube of siphon; f, foot; fo, foot opening of 
mantle; g, gills; 7s, walls of incurrent siphon opening; widely opened, the incurrent 
siphon opening becomes one with the foot opening. 
Fig. 31 Modiolaria nigra; part of figure 30 enlarged. 
Fig. 32 Semele decisa; ap, anterior palp; es, excurrent tube of siphon; 7s, 
incurrent tube of siphon; other letters as in figure 27. 
JOURNAL OF MORPHOLOGY, VOL. 26, NO. 4 
