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 as in M. margaritifera, and thus the structure 

 of the diaphragm is essentially the same. 



Both gills are quite delicate. The two 

 laminae 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 

 about the arrangement of the ova in the gills, 

 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 

 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 



Fig. 2a. Left gills of 

 another specimen from 

 same locality. 



