320 ANNALS OF THE CARNEGIE MUSEUM. 
and the peculiar posterior point are quite evident in all three specimens, 
and the same is true of the peculiar curve of the ovisacs, so that these 
features cannot be accidental. Glochidia higher than long. Length 
0.20; height 0.22 mm. (see Plate XIX, fig. 8). 
Colors largely faded in my alcoholic material. One specimen has a 
peculiar brown line across the middle of the foot. The edge of the 
marsupium has brownish black pigment in spots. 
This species and genus is one of the most peculiar with which I am 
acquainted. I would have considered it a Nephronajas, but since I 
have investigated another Mexican Nephronajas (see below), it is 
clear it cannot belong to this genus. The generic name is selected in 
recognition of the valuable help in my work received from Mr. L. S. 
Frierson. 
Genus OBOVARIA Rafinesque. (1819.) 
(Simpson, 1900), p. 599.) 
Shell rounded or ovate, higher than long, or only slightly longer 
than high, inflated, without distinct posterior ridge. Disk not sculp- 
tured. Beak-sculpture poorly developed, consisting of few subcon- 
centric bars, of which the later ones have sometimes the tendency to 
become sinuate, but are not distinctly double-looped. Epidermis 
yellowish to brownish, rarely greenish, with indistinct, simple rays 
or without rays. Male and female shell slightly different in shape, 
the female being generally a little expanded on the post-base, but this 
difference is sometimes hardly noticeable. 
Inner lamina of inner gills entirely connected with abdominal sac. 
Edge of the mantle very little differentiated in front of the branchial. 
It is slightly lamellar, with fine crenulations; and this part is generally 
emphasized only by the thickening of the margin of the mantle and the 
presence of a streak of dark pigment; there are never papille on it. 
Marsupium consisting oi many ovisacs, occupying the posterior part of 
the outer gill. The ovisacs, when charged, swell transversely, so as to 
become lanceolate and compressed. They reach from near the base 
of the gill to, and a good deal beyond, the edge of the gill, and the whole 
marsupium assumes a kidney-shape. Placentz not very solid. Glo- 
chidia all through the placental mass, of medium size and subovate. 
Type O. retusa (Lamarck). 
This is another primitive type of the Lampsiline, leading, however, 
toward the more highly developed forms of the subfamily. The 
