512 MEMOIRS OF THE CARNEGIE MUSEUM. 



supiuni (PL XLVI, fig. 2) it is to be remarked that it occupies in larger specimens 

 a somewhat greater portion of the inner gill, leaving anteriorly as well as posteriorly 

 less than one-fourth of the gill free. In smaller females the marsupial part is 

 relatively smaller. The interlaminar connections form distinct-, but interrupted, 

 septa. The connections stand verj' close, and the septiform structure i:)revails 

 throughout the marsupium, and very little is seen of a transverse or reticulate 

 arrangement. In some parts, chiefly towards the margin, the septa become more 

 or less continuous. Thus there are here rather distinct, but intercommunicating 

 water-tubes (ovisacs), which are filled, when charged, with masses of eggs cormected 

 with each other through the communications, so that no placenta-like arrangement 

 is observed. 



Glochidium (Text-fig. 4/?, p. 469). — Higher and more upright than that of D. 

 charruanus; L, and H. about the same, 0.28 to 0.29 mm. Thus it is more like that 

 of D. firmus figured by Lea (Obs., XII, 1869, PI. 34, fig. 82), but not quite as upright. 

 At the point of the lower margin there is a hook of the usual shape, about 0.09 mm. 

 long. But such hooks are present in only one of my specimens, in the other the 

 glochidia are too immature. 



It would be quite remarkable if in two species, so closely allied as D. charruanus 

 and piceus, the glochidia should differ so fundamentally, that in one there are 

 hooks, and in the other not. But judging from my material, this is the case. How- 

 ever, it must be emphasized again that my material is scanty, and possibly in the 

 case of D. charruanus I do not at all have ripe glochidia. 



16. DiPLODON URUGUAYENSis (Lea) (1860). 



Unto uruguayensis Lea, Obs., X, 1863, PL 45, fig. 298. 



Diplodon uruguayensis Simpson, 1914, p. 1234. 



Unto apprimus Lea (1866), Obs., XII, 1869, PL 33, fig. 78. 



Type-locality. — Uruguay River. 



New Locality. — Pond along Rio Negro, Santa Isabel, Uruguay (J. D. Haseman 

 coll., February 11, 1909). Live complete specimens, and several odd valves, 

 two specimens (male and female) with soft parts. 



Distribution. — Known only from Rio Uruguay and Rio Negro. 



The sjTionymy and affinity of this species is obscure. Simpson thinks that it 

 is close to D. wymani, but the latter is a compressed shell, while uruguayensis is 

 much swollen. U. apprimus also is a swollen shell. Lea has already suggested 

 that this is close to uruguayensis, Init that it differs chiefly in the hinge-teeth, 



