496 



MEMOIRS OF THE CARNEGIE MUSEUM. 



3. The lower margin of the shell of D. swiillimus is frequently more nearly 

 straight, so that the shell of older si)ecimens appears more humped. 



4. Color of epidermis with hardly any groon tints, but light to dark lironze- 

 brown, in old shells brown-black. 



5. Pscudocardinal teeth never stumpy, chiefly in left valve, but always com- 

 pressed, although the two of the left valve and the posterior in the right are rather 

 thick in old specimens. 



Measurements. 



The characters of the shells of this species are rather poorly marked, and can 

 be ascertained only by the examination of extensive material. I have discovered, 

 however, that there are important and constant differences in the anatomy. 



Anatomy. — Fully agreeing with that of D. imitator, but the marsupium is 

 different (See PI. XLV, fig. 2). It is located in the anterior part of the inner gill, 

 entirely anterior to its middle, and extending forward to within a short distance 

 of the anterior end of the gill, so that anteriorly less than one-tenth of the gill is 

 non -marsupial, while posteriorly fully the posterior half of it is non-marsupial. 

 The interlaminar connections form interrupted septa, but the septiform structure 

 is less distinct than in D. imitator, and more of a reticulated (or irregular quin- 

 cuncial) arrangement is evident. This structure of the marsupium is the same in 

 all females investigated (altogether twenty individuals), and only in young ones is 

 the marsupial part smaller. 



The Glochidium (Text-fig. Ac, p. 469) differs from that of D. imitator in being 

 more distinctly oblique, and being longer than high. L. 0.28 mm., H. 0.24 mm. 

 There are hooks of the same type, which are about 0.10 mm. long. 



My specimens were collected on January 3, which date indicates the breeding 

 season, probably its middle, for eight females had only eggs, while nine had glochidia, 

 in part not mature, and with the hooks yet unformed. 



The remarkable fact brought out by the study of the anatomy is that, while 

 the species is extremely hard to distinguish from imitator by the f<hcll, it has at least 

 tiro anatomical characters (marsupium and glochidium), ivhich are very well marked 

 and constant. 



