660 JAMES LL. KELLOGG 
figures 30 and 31. In the latter case, water enters the foot 
opening as freely as the incurrent siphon opening, as proved by 
the use of carmine. The sexual glands fill the mantle as in 
Mytilus. I have presumed (92) that this shifting in the position 
of the sexual mass in lamellibranchs is often due to the fact that 
the byssus muscles come to occupy so much of the space of the 
visceral mass. Modiolaria has a well developed byssus, and a 
foot with a byssus groove. 
The ciliation of the body is much like that of Mytilus. Gill 
currents on all lamellae are to the margins, and forward to the 
palps. These latter organs are small and hard to examine, but 
ciliation on them seemed to be as in Mytilus. On the mantle 
walls, material is carried ventrally and then posteriorly. It 
continues on its backward course, over the dorsal wall of the 
ineurrent siphon (7s) to the exterior, as shown in figures 30 and 
31, against the entering current of water; and unless we except 
Mytilus where the anatomical features of the mantle are not the 
same, this performance is unique among lamellibranchs examined. 
It was observed that the effect of the stimulus of a large amount 
of carmine or sand in the water, was to open wide the connection 
between the incurrent siphon and foot openings; and that while 
water entered the mantle chamber freely, a steady stream of 
waste matter poured out over the dorsal wall of the incurrent 
siphon. 
Semele decisa Conrad 
One small specimen from San Diego Bay was examined. ‘The 
courses of the chief cilia currents are shown in figure 32. 
Macoma secta Conrad 
This genus is one of the most interesting of those examined on 
account of its unique habit of taking large quantities of sand 
into the digestive tract. Some of its five or six species are found 
from Alaska to Mexico on the Pacific coast. Macoma secta 
(fig. 33), the largest, hasa shell 7 or8 cm. in length. Members of 
the genus burrow several inches, and have the habit of lying on 
one side. The posterior edges of the shell valves, where the two 
