CILIARY MECHANISMS OF LAMELLIBRANCHS 687 
backward prolongation of the visceral mass. Whether col- 
lections accumulate on its ventral surface or not, could not be 
determined. Because the course of cilia currents over the 
entire visceral mass is so much like that of B. costata, it would 
seem that there is here a much degenerated collecting membrane, 
which formerly may have been much more extensive. 
Mantle. The walls of the mantle chamber, unlike those of 
Zirfaea, may be distinguished from siphon walls, because they 
are very thin, while the siphon walls are muscular. Currents 
are directed ventralward from the sides to the fused ventral 
margins. The details of currents in this region are shown in 
figure 62 which represents a view of the fused ventral man- 
tle edges seen from above. The foot opening (fo), through 
which the disk-hke end of the visceral mass may protrude 
slightly, is still large. Some of the material brought from the 
side walls of the mantle, pauses, and may collect in small balls 
at two points (v) which suggest the vortices of Mya. There 
seems to be no whirling movement here, and collections are 
sooner or later moved on to the thickened and fused edges. At 
several places, as shown by the arrows, they pass on to a median, 
broad, slightly depressed tract (me) and are carried swiftly 
backward to a point, b, corresponding in position to the bay at 
the base of the incurrent siphon, which has been shown to be so 
generally present on the lamellibranch mantle. 
Unio complanatus Solander 
Gills. Specimens were from a lake in western Massachusetts. 
On both lamellae of the outer demibranch (fig. 63, og) collections 
are carried to the base, where, on each side, they are moved 
anteriorly, along narrow lines, to the distal oral groove, just in 
front of, and parallel with, the anterior margin of the inner 
demibranch (fig. 64, dg). Unless the material is very small in 
amount, it touches the anterior wall of this groove made by the 
mantle (m), and is lifted out and carried away by the mantle 
cilia. On both lamellae of the inner demibranch, material is 
carried to the free margin, and then forward in its groove to a 
