250 Annals of the Carnegie Museum. 



ture of the septa and the epithelium, which is folded and wrinkled as 

 usual in marsupial gills. 



Soft parts whitish. Frierson writes to me that in this species "eggs 

 are in two gills sometimes, mostly in all four." Of those sent to me 

 (five females), the structure of the gills was alike, in every case all 

 four gills had the marsupial structure as described above. 



In this species I see the nearest approach to a tendency to restrict 

 the marsupial function to the outer gills, in so far as the water-tubes 

 are slightly wider in the inner gills. But still the latter partake in the 

 formation of the marsupium, and are distinctly marsupial at least in 

 their marginal half. 



This species stands rather isolated also with regard to the characters 

 of the shell, but the features of Crenodonta are clearly seen, and I think 

 that the most closely allied form is C. heros. 



Genus Quadrula Rafinesque. 1820. 

 Simpson, 19006, p. 765 (restricted). 



Shell rounded, quadrate, or subrhomboidal, sometimes elongated, 

 with rather high beaks, well developed hinge-teeth, and deep beak- 

 cavities. Outer surface more or less sculptured, with tubercles, 

 pustules, or ridges, but without the characteristic oblique folds of 

 Crenodonta. Epidermis lighter or darker, generally with rays, and 

 often with beautiful color-patterns caused by the breaking up of the 

 rays. Beak-sculpture concentric, double-looped, or zig-zag, poorly 

 developed, or extending upon part of the disk. 



Soft parts primitive in structure. Supra-anal separated from the 

 anal by a short mantle-connection, the latter sometimes absent. 

 Inner lamina of inner gills free. All four gills marsupial (see Plate 

 XVIII, fig. 1), but the water-tubes of the inner gills sometimes a little 

 wider than those of the outer gills, although this difference is occasion- 

 ally hardly noticeable. When gravid, the ovisacs expand moderately, 

 giving a compressed, leaf-like shape to the placenta?. Placentae 

 (where known) whitish, not very solid, and not persistent. Glochidia, 

 in the few cases known, small, or medium, subovate, without hooks. 



Type Q. metanevra (Rafinesque). 



The species belonging to this genus may easily be separated into 

 three groups: 



1. Pustulosa-group. 



Shell more or less rounded and swollen over the disk, with pustules 



