234 ANNALS OF THE CARNEGIE MUSEUM. 
riorly. Anterior ends of the two gills as in M. margaritifera. Outer 
lamina of outer gill not entirely connected with the mantle: posteriorly 
a part of it is free (about one-seventh of length of gill, but probably 
more, since this part in all my specimens is considerably contracted). 
Inner lamina of inner gill asin M. margaritifera, and thus the structure 
of the diaphragm is essentially the same. 
Both gills are quite delicate. The two 
lamine are not connected by septa running 
parallel to the gill-filaments, but the inter- 
laminar tissue forms septa of another type: 
they run obliquely, diagonally, from the base 
of the gill downward and forward. Of these 
septa some are longer, others shorter, and 
toward the edge of the gill they sometimes 
curve a little in the direction of the filaments. 
The septa thus are rather irregular, and being 
quite distant from each other, no regular 
water-tubes are formed. 
A difference in the arrangement of the 
septa, which might be due to sex, could not be 
observed in the specimens at hand. None of 
them was gravid, so that nothing can be said 
wither anacimen’ from about the arrangement of the ova in the gills, 
same locality. and about the glochidia. The fact, that in all 
specimens the structure of the two gills is 
practically identical, suggests, however, that all four gills are used as 
Fic. 2a. Left gills of 
marsupia. 
Color of soft parts whitish, edge of mantle blackish all around, but 
chiefly at the anal and branchial openings. Gills transparent, but 
not blackish. Foot grayish-white in its distal part, this gray color 
marked off in a sharp line from the basal white part. 
M. monodonta agrees in most characters with M. margaritifera, and 
chiefly in the general form of the margin of the mantle, the branchial 
and anal openings, the diaphragm, and the structure of the gills. The 
chief difference is found in the diagonal, incomplete septa of the gills, 
which, however, unmistakably correspond to the irregular diagonal 
rows of interlaminar connections in M. margaritifera. In this respect, 
M. monodonta represents a stage of development slightly more in 
advance of that of M. margaritifera, and this would support the view 
