330 Annals of the Carnegie Museum. 



fourths of the length of abdominal sac, the minimum was only a small 

 hole at the posterior end of the foot, but this was only on one side of 

 the body; on the other side the lamina was free for a little less than 

 half the length of the abdominal sac. 



Gills of the usual structure. Marsupium kidney-shaped when 

 charged, occupying the posterior half or more of the outer gill, with 

 numerous (thirty and more) ovisacs, of the Obovaria-type. Ovisacs 

 compressed, lanceolate, with poorly developed placentae. Glochidia 

 distributed all through the placentae, of unusual shape (see Lea, Obs., 

 VI, 1858, pi. 5, fig. 6; 46 Lefevre and Curtis, 1910, p. 97, fig. H; Ort- 

 mann, 191 lb, pi. 89, fig. 17). They are quite large, subspatulate, 

 (dilated and rounded off toward the ventral margin), and their an- 

 terior and posterior margins are distinctly gaping. Lefevre and Curtis 

 give the measurements as follows: length 0.23; height 0.31; while my 

 maximum measurements are: length 0.26; height 0.35 mm. 



Color whitish. Edge of mantle brownish black, chiefly so in the 

 posterior region. The black pigment is emphasized along the edge 

 in front of the branchial. 



Genus Paraptera Ortmann. (191 1.) 

 (Ortmann, 191 lb, pp. 301, 334, 338.) 



Shell thin, elliptical, or obovate, when young with a distinct pos- 

 terior wing, rather compressed, without posterior ridge. Disk without 

 sculpture. Hinge-teeth feebly and often imperfectly developed. 

 Beak-sculpture fine, consisting of a few concentric bars, followed by a 

 few otheis, which are double-looped. In the latter, only the posterior 

 loop is distinct, while the anterior is obliterated. Male and female 

 shells slightly different, the female shell more expanded at post-base. 



Inner lamina of inner gills entirely connected with abdominal sac. 

 Edge of the mantle of the female slightly lamellar in front of branchial, 

 with crenulations, but not with papillae. Marsupium kidney-shaped, 

 swollen, consisting of many ovisacs occupying the posterior part of 

 the outer gills. Placentae not very solid. Glochidia very small, of 

 suboval shape. 



Type P. gracilis (Barnes). 



Another genus having the structure of Obovaria, distinguished only by 

 the shape of the shell and the glochidia. The latter are very remark- 

 able, and can only be compared with those of Amygdalonajas. 



46 This figure is not drawn to scale, and is much too small. 



