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, 1900), 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 placente. Placente 
(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 
