322 



Annals of the Carnegie Museum. 



with fine crenulations. It is denned on the inner side by a narrow 

 stripe of black pigment. In the male this lamella is also present, but 

 much weaker. Farther in front the edge of the mantle is smooth. 

 Palpi small, normal, their posterior margins connected only at the 

 base. 



Gills short and broad, the inner ones broader. Diaphragm normal. 

 Inner lamina of inner gills entirely connected with abdominal sac. 

 Anterior attachment of gills as usual. 



Septa and water-tubes in both gills normally developed. Mar- 

 supium restricted to a small section in the posterior half of the outer 

 gill, leaving more than half of the anterior portion and a small posterior 

 section non-marsupial. Ovisacs fifteeen to twenty (in my specimens) ; 

 when charged not narrower than the normal water-tubes in the longi- 

 tudinal direction, but expanding in the transverse direction, so that 

 their lumen becomes lanceolate and compressed, the whole mar- 

 supium thus appearing swollen and kidney-shaped. The marsupium 

 extends considerably beyond the original edge of the gill, and about 

 three-fourths of the length of the ovisacs is within the laminae of the 



Fig. 20. Obovaria retusa (Lamarck). Gravid female, from Ohio River, Port- 

 land, Meigs Co., O. (Cam. Mus., No. 61, 4.773-) 



gill, while one-fourth lies beyond the latter. Edge of marsupium blunt, 

 without pigment. Placentae not well developed; eggs and glochidia 

 rather loose. 



Glochidia rather large, suboval, without hooks. Length 0.22; 

 height 0.27 mm. (see Plate XIX, fig. 9). 



Color of soft parts whitish, only edge of mantle brown, with a black 

 streak in front of the branchial. Charged marsupium whitish. 



