CILIARY MECHANISMS OF LAMELLIBRANCHS 663 
and collected at the base of the incurrent siphon. Some of the 
sand in the mantle chamber is free, but much of it is cemented 
together by mucus from mantle, gills, and palps, and certainly 
the mechanism exists which may carry it all out of the body. 
Nevertheless, freshly opened specimens show so much sand cover- 
ing all organs in the mantle chamber, that the outgoing tracts, 
if operating as in other forms, would prevent any material what- 
ever from entering the mouth. It is impossible to believe that 
there has been any error in interpreting the outgoing tracts of 
lamellibranchs as mechanisms adapted for cleaning the mantle 
chamber of an excess of material which is usually undigestible 
and undesirable when the water is muddy, the object being to 
prevent its entrance into the digestive tract. Yet in Macoma 
enormous quantities of silt and sand do enter the digestive tract, 
and the failure of the outgoing system to operate as in other forms 
is as yet without explanation. It is a problem worthy of all the 
time that may be needed for its solution. 
The tremendous power of cilia was especially well observed 
here, though perhaps really not greater than in other bivalves. 
The gills of Macoma, covered with a layer of sand a millimeter 
or more in depth, completely cleared themselves in the course 
of two minutes. 
Monia machrochisma Desh 
The rounded shells of the specimens studied at Puget Sound 
were more than 7 em. in diameter. The form is particularly 
interesting on account of the extreme distortion and asymmetry 
of the soft parts of the body, and the shifting of several organs 
from their original positions. Probably on account of this, 
there have occurred reversals of some cilia currents from the 
usual course and the addition of others not found elsewhere, un- 
less perhaps in Anomia, which has not been studied. 
The left valve, as in Anomia, and to a less degree in Ostrea, 
is large and saucer-like, while the right is smaller and flat, and 
contains a deep bay through which the attaching byssus is 
extended. Figure 34 represents the soft parts, except the gills 
of the right side, when the right valve is removed. The region 
