336 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 papille are black, the black color extending broadly 
upon the inside of the mantle. 
Fic. 22. Medionidus conradicus (Lea). Gravid female, from South Fork of 
Cumberland River, Burnside, Pulaski Co., Ky. (Carn. 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. 411).” 
Genus EuRYNIA Rafinesque. (1820.) 
Simpson, 1900), 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 Jamina 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 papilla, 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. 
®2 A byssus-thread is frequently found in young. Unionide, 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). 
