498 



MEMOIRS OF THE CARNEGIE MUSEUM. 



Characters uf the Shell. — The dcscriiition of D. itnitator miglit also serve for 

 this species. However, the following differences are noticeable: 



1. Posterior ridge of shell broad, and more or less distinctly biangulate, pro- 

 ducing a biangulation also of the posterior end of the shell. This is the most 

 striking character of the shell. In both D. imitator and simillimus the posterior 

 end of the shell is evenly rounded without any trace of angulations. 



2. Epidermis not so shining as in the two other species, which is due to the 

 development of additional fine concentric wrinkles, which are irregular and best 

 developed upon the posterior slope and near the lower margin. Upon the disk 

 they form indistinct radiating lines (or narrow bands), which are obsolete in the 

 two other species. Color of epidermis of vican'us lighter or darker brown, without 

 green tints, and without bronzy lustre. 



The hinge-teeth are generally of the type of D. simillimus, i.e., they do not 

 become stumpy in old shells. They are, however, very variable, and much cut 

 up, more so than in the two other species, and the posterior pseudocardinal of 

 the left valve frequently is quite small or rudimentary. The nacre is whitish, but 

 often inclines to lurid tints (purplish gray). 



Measurements. 



No sexual differences in the shell. 



Anatomy. — I have eight females, six of which are gravid; three with eggs, three 

 with glochidia (immature in one). The structure of the soft parts is exactly as in 

 the preceding species, except that of the marsupium (Plate XLV, fig. 3). As in 

 D. simillimus, the marsupium is located in the anterior portion of the inner gill, 

 anterior to the middle, but it occupies a still smaller part, and does not extend so 

 near to the anterior end, and does not reach the middle of the gill. Anteriorl}^ 

 about one-ninth of the gill is non-marsupial, and posteriorly over half of it. The 

 interlaminar connections are arranged in interrupted septa, the septiform arrange- 

 ment being most evident anteriorly and in the middle of the marsupium, while 

 posteriorly the arrangement is reticulate (indistinctly quincuncial). 



The glochidium (Text-fig. 4rf, p. 469) is similar to that of D. simillimus, oblique, 

 but slightly longer. L. 0.30, H. 0.24 mm. There are hooks of the same type, which 



