494 MEMOIRS OF THE CARNEGIE MUSEUM. 



the species (observed in seventeen females), with the only qualification that in 

 young females the marsupium is smaller (less than two-fifths of the gill) but has 

 a similar position. 



Marsupial part formed by interrupted septa (PI. XLV, fig. 16; PI. XLVII, 

 fig. 6). The arrangement into septa is distinct, but they are frequently inter- 

 rupted, and their solid portions are short. In the middle of the marsupium they 

 are at the utmost six to eight times as long as thick, and in its other parts they are 

 very short. A distinct quincuncial (reticulate) arrangement is not seen. Toward 

 the margin of the gill the septiform arrangement is again more distinct. 



The embryos fill the marsupium in an irregular mass, and the charged mar- 

 supium is moderately swollen. Placentae are not formed. 



Glochidium (text-fig. Ah, p. 409, observed in six specimens) subtriangular, 

 slightly oblique, anterior and posterior margins convex, converging to a point, 

 anterior margin longer than the posterior. Hooks present, of the Hyriine type, with 

 the S-shaped curve. L. 0.27 to 0.28 mm. ; H. 0.27 to 0.28 mm. ; L. of hook: 0.09 mm. 



It should be remarked that in one female the glochidia were immature, and 

 no hooks could be seen; three females had only eggs, and one female was in the act 

 of discharging. The dates of collecting, Jan. 26 and 29, should be recorded as 

 showing the breeding season (midsummer of Southern Hemisphere). 



Remarks on the Speeific Characters. — ^As will be seen from the descriptions 

 of the following species, D. simillimus, D. vicarius, and D. decipiens are very closely 

 allied to the present species in the shape of the shell; vicarius can be readilj' dis- 

 tinguished by the biangular posterior ridge, and D. simillimus is a smaller shell. 

 In other respects it is almost impossible to distinguish these species by the shell 

 alone. The few obscure, differentiating characters will be pointed out below under 

 the respective species. However, the investigation of the soft parts has shown 

 that there are interesting differences chiefly in the location of the marsupium. In 

 the present species, the marsupium is most fully developed, occupying a rather 

 large part of the inner gills, slightly gravitating toward the anterior end. In the 

 following species (chiefly simillimus and vicarius) it will be seen that this tendency 

 is increased, and the marsupium becomes smaller, and is being shifted more dis- 

 tinctly forwards (Compare pi. XLV, fig. 16, with PL XLV, figs. 26, and 3). It 

 will also be seen that there are slight differences in the glochidia with regard to 

 obliquity and size. 



