Ortmann: Families and Genera 01 Naiades. 335 



Inner lamina of inner gills free from abdominal sac, except at an- 

 terior end. Inner edge of mantle in front of branchial, with a series 

 of rather distant, thin, but long, papillae, resembling thick hairs. Mar- 

 supiurn formed by comparatively few ovisacs (six to eight), swollen, 

 kidney-shaped, and occupying only a small part of the outer gill, im- 

 mediately behind the middle. Ovisacs rounded at the end, projecting 

 beyond the edge of the gill, not very sharply marked off from each 

 other. Placentae not solid. Glochidia subovate. 



Type .1/. conradicus (Lea). 



This is a doubtful genus. The description of the marsupium of 

 M. parvulus (Lea, Obs., XI, 1867, p. 45) is similar to that of the type- 

 species. In M. penicillatus, Lea (Obs., VII, i860, p. 203) describes 

 the marsupium as larger, but in a foot-note he mentions that in 

 another specimen it was as small as that of conradicus. In M. 

 acutissimus, the marsupium occupies, according to Lea (Obs., X, 1863, 

 p. 411), the whole length of the gill. Simpson also says in the generic 

 diagnosis, that the marsupium sometimes extends nearly the whole 

 length of the gill. If thus the shape of the marsupium as described 

 above should be inconstant, the most important generic character 

 would be taken away, and Medionidus would fall as a synonym under 

 Eurynia, with which genus it is indeed closely related in all other 

 respects. Possibly in this case it would be best to make it a subgenus 

 of Eurynia, on account of certain other peculiarities, such as the free 

 inner lamina of the inner gills and the shell-sculpture. 



Medionidus conradicus (Lea). 



A male and a gravid female have been sent me by B. Walker, from 

 the South Fork of the Cumberland River, Burnside, Pulaski Co., 

 Kentucky. 



Anal and supra-anal separated by a mantle-connection of medium 

 length. Anal crenulated, branchial papillose. In front of the branchial 

 the edge of the mantle carries in the female distinct, long, slender, hair- 

 like papillae almost as far forward as the middle of the lower margin. 

 These papillae are also present in the male, but considerably shorter. 

 Palpi with the posterior margins connected at the base. 



Diaphragm and gills of usual shape. Marsupium small, consisting 

 of six to eight indistinctly separated ovisacs, forming an almost 

 globular swelling just behind the middle of the outer gill. Glochidia 

 rather large, subovate, or almost subspatulate. Length 0.22; height 

 0.28 mm. (see Plate XX, fig. 3). 



