33(> Annals of the Carnegie Museum. 



Color of soft parts whitish. Margin of mantle brown all around, 

 most intensely posteriorly. The edge of the mantle in front of the 

 branchial and the papillae are black, the black color extending broadly 

 upon the inside of the mantle. 



,sa 



an 



PP 

 mp 



Fig. 22. Medionidus corn-adieus (Lea). Gravid female, from South Fork of 

 Cumberland River, Burnside, Pulaski Co., Ky. (Cam. Mus., No. 61, 4,989.) 



Both specimens at hand have a distinct byssus (the male is 18 mm., 

 the female 27 mm. long). Compare Lea (Obs., X, 1863, p. 410; and 

 M. acutissimus, ibid., p. 41 1). 52 



Genus Eurynia Rafinesque. (1820.) 

 Simpson, 19006, p. 534 (as subgenus). 



Shell subelliptical, often rather elongated. Outside of shell not 

 sculptured. Beak-sculpture of the double-looped type, rarely sub- 

 :oncentric, often quite obsolete. Epidermis generally yellowish or 

 greenish, with more or less distinct rays, rarely darker and blackish. 

 Shell of the female quite distinct from that of the male, more or less 

 swollen, or expanded in the post-basal region. 



Inner lamina of inner gills generally wholly connected with abdominal 

 sac, rarely more or less free. In the female the inner edge of the mantle in 

 front of the branchial has always distinct papillae, which may be large 

 or small, more or less numerous, and differently arranged. In the 

 male a similar structure is observed, but in a rudimentary condition. 



52 A byssus-thread is frequently found in young Unionidce, as observed by 

 various authors and myself. This is undoubtedly a real byssus. Whether it is 

 in any way connected with the embryonic "byssus" or larval thread, remains to 

 be ascertained. According to Lillie (1905, pp. 52-54) the latter is not homologous 

 to the byssus of other Lamellibranchiata, and is a larval organ serving originally 

 the function of excretion and secondarily the function of attachment. In Medio- 

 nidus, the byssus seems to be almost regularly present, and to be persistent. In 

 other species (Nephronajas ligamentina, Lampsilis ventricosa, and others) where I 

 have seen it, it is present only in young specimens. See also Isely (1911, p. 77). 



