314 Annals of the Carnegie Museum. 



this part is very small when compared with the prolonged portion. 

 The latter curves backward in a circle, and is rolled up spirally, the 

 spiral forming about one and a half to two turns, but only the posterior 

 ovisacs complete the whole revolution, while the anterior ones stop 

 earlier, the first after completing the circle about once. The distal 

 parts of the spiral wind up in the direction toward the median line of 

 the body, so that in a view from the outside, they are hidden under 

 the outer gill and the first whorl of the marsupium. 



The ova fill the ovisacs in the shape of closely packed masses, forming 

 distinct and very solid placentae, red in color, rarely white. Glochidia 

 rather small, almost semicircular, distinctly longer than high, without 

 hooks. Length 0.18; height 0.15 mm. (see Plate XIX, fig. 6). Sterki 

 (1898, p. 19) gives the dimensions as length 0.21 ; height 0.17; diameter 

 0.14 mm. He also says that the glochidial shell is "considerably 

 longer than high and has numerous distinct, crowded, concentric lines 

 of growth." I have not seen the latter. The shape of the glochidia 

 approaches to a degree, that of Dromus, but the disproportion between 

 length and height is much less. 



Color of soft parts whitish. Abdominal sac and mantle suffused 

 with black. Edge of mantle brown with black spots, this mottling 

 extending all around. Marsupium, when charged, red, or (according 

 to Sterki) sometimes white. 



Genus Dromus Simpson. (1900.) 

 (Simpson, 1900&, p. 614.) 



Shell very much like that of Cy progenia. Beak-sculpture obsolete, 

 described by Simpson as consisting of interrupted, concentric ridges, 

 but I have never seen them distinctly. 



Inner lamina of inner gills partly free from abdominal sac, connected 

 near the anterior end for about one-third, or more, of the length of 

 the abdominal sac. Edge of the mantle in front of branchial without 

 special structures. Marsupium consisting of numerous ovisacs, which 

 occupy the larger posterior portion of the outer gill, leaving a smaller 

 anterior section non-marsupial. The ovisacs are comparatively short, 

 subcylindrical, or only slightly compressed, and lie practically entirely 

 beyond the original edge of the gill. In older individuals, the mar- 

 supium becomes warped and folded. Placentae solid, subcylindrical, 

 or slightly compressed, rather short. Glochidia placed chiefly toward 



