UNIO 529 
having the young only in the outer gills. Those forms that 
have all four of the gills used as a marsupium have generally 
shorter, solider, more inflated shells than those with the em- 
bryos in the outer gills alone, and their beak sculpture is 
coarser and approaches more nearly to the radial pattern. 
The Oriental forms with zigzag beak sculpture and the South 
American species whose beaks are marked with radial bars, 
all of which, I believe, contain the young in the inner gills 
alone, are still wider separated from the true Unios, even 
though they may have perfect teeth. 
In so large and widely distributed a group as even the 
restricted genus Unio there is much diversity of form and 
structure. Generally the shell is more or less elongated, but 
such species as U. littoralis and some of its allies are short, 
being subrhomboid or nearly orbicular. Unio crassidens is 
sometimes as ponderous as any of the species of Quadrula. U. 
pictorum and some related forms have a bright, even some- 
what rayed, epidermis. There is much diversity in the beak 
sculpture and some forms have a small upper pseudocardinal 
in the right valve. 
Section LymMNium Oken, 1815. 
Lynmnium OxKen, Lehrbuch, 1815, p. 237. 
Shell generally smooth; beak sculpture broken, often some- 
what corrugated or pustulous; pseudocardinals compressed ; 
beak cavities well excavated, not compressed. Animal highly 
colored; anal opening crenulate or smooth. 
Type, Unio pictorum Linnzus. 
Group of Unio pictorum. 
Shell inflated, elongate, oval; anterior end angled above, 
swollen a little at posterior base; beaks full, their sculpture con- 
sisting of numerous slightly doubly-looped bars, which often 
become pustulous; posterior ridge rather low; epidermis 
smooth, rather bright, sometimes slightly rayed behind; rest 
periods well marked; pseudocardinals compressed, often a lit- 
