534 MEMOIRS OF THE CARNEGIE MUSEUM. 



Anatomy. — It was not possible to positively ascertain the sex of my smallest 

 specimens. But, I have other s])ecimens, of which five are positively males, and 

 seven of which are females. None of the latter were gravid. 



Color of soft parts whitish, distal part of foot brown. 



Anal opening closed above; closed part considerably longer than the open 

 part; the latter slit-like, shorter than the branchial opening, and separated from 

 it by a solid mantle-connection. Branchial opening with distinct papilliie. Palpi 

 subtriangular, their jjosterior margins connected for about one-third of their 

 length. 



Gills of the usual shape, rather wide, the inner anteriorly wider, its anterior 

 end close to the palpi. Inner lamina of inner gill entirely connected with abdominal 

 sac. Non-marsupial gills of the usual structure, with few and scattered inter- 

 laminar connections. Marsupvum of the female (PI. XLVI, fig. 7b) located in 

 the inner gill, occupying nearly half of it in the middle, leaving non-marsupial 

 one-fourth of the gill both at the anterior and posterior end. In young females 

 the marsupial part is smaller (PL XLVI, fig. 7a), but also located nearly centrally. 

 Interlaminar connections of the marsupium reticulated or quincuncial toward the 

 base, but forming interrupted septa toward the margin. 



25. DiPLODON GRATUs (Lea) (1860). 

 Section of gills: Plate XLVIII, fig. 3. 

 Unio (jratm Lea, Obs., X, 1863, PI. 43, fig. 290; Sowerby, XVI, 1868, PL 84, fig. 444. 

 Diplodon (Cyclofnya) gratus Simpson, 1900, p. 886. 

 Diplodon (Cyclomya) fontainianus gratus Simpson, 1914, p. 1281. 



Type-locality. — Rio Uruguay. 



New Locality. — Rio Uruguay (in mud), Uruguayana, Rio Grande do Sul, 

 Brazil (J. D. Haseman coll., February 5, 1909) twenty-one specimens, males 

 and females, all with soft parts. 



Distribution. — Positively known only from the Uruguay River. Von Ihering 

 (1893, p. 92) gives this form also from Rio Guahyba, Porto Alegre, Rio Grande 

 do Sul. However, as we shall see, this is not the typical gratus, and the Guahyba- 

 form has already been distinguished by Simpson as var. deceptus (see below). 



Description of Shell. — Rather small, maximum length of my specimens 53 mm., 



moderately thick. Outline obliquely subrotund, subovate, or subtrapezoidal, 



very variable, more or less high (height 72 to 85 pr. ct. of length). There is an 



indistinct relation of the shape to sex, in so far that the highest shells (79 pr. ct. 



.and over in height) are all males, while the more elongated specimens may be either 



