1895.] On the Validity of the Genus Margaritana. 343 
duced, and the teeth and plate are badly injured. And the 
erosion of the beaks so common to Naiads in many streams 
usually damages the teeth. 
There seem to be two forms of hinges among the so-called 
Margaritanas, the one like that of M. margaritifera and holstonia, 
in whichthe area occupied in the Unios by the laterals is 
smooth and destitute of teeth, the other like that of M. com- 
planata and calceola in which the laterals are badly blurred 
and broken up as if by disease; the plate being covered with 
long, low, irregular ridges which run somewhat diagonally 
across it from the region of the beaks towards the interior of 
the shell. I would suggest that different causes may have 
operated to produce these different conditions. Dr. Dall holds, 
and I believe with good reason, that the teeth of bivalve shells 
are developed for the purpose of keeping the valves in their 
proper place. In such cases as they interlock it is well-nigh 
impossible that one valve should be twisted out of place with- 
out injuring the animal or its shell. Nearly all Unios which 
have strong, perfectly developed teeth live in running water, 
often in rapid currents, in fact it is well known that the Unios 
are more generally inhabitants of streams and rivers while the 
Anodontas, which have no teeth, live as a rule in ponds or 
other still waters. The different species of Cristaria, in which 
the teeth are reduced to mere rudiments, live in ponds and the 
ditches of rice fields, in the mud. Unio hembelii, with very 
faint laterals is found in the sluggish bayous of Louisiana. 
Margaritana monodonta, which is often nearly destitute of teeth, 
though living in rivers is almost invariably found under stones 
in mud, as is M. hildrethiana; both of them therefore being 
protected from currents. I think that the want of teeth in such 
forms can be explained by supposing that they have degener- 
ated on account of their being no longer needed. 
Such species as Margaritana confragosa, rugosa, complanata 
and the like, which have blurred or distorted teeth usually 
are found in running water, often in rapid streams, and I am 
inclined to believe that they are forms which are peculiarly 
susceptible to injurious influences, and that their teeth have 
become diseased on account of these influences. And it seems 
