342 The American Naturalist. [April, 
It would be very interesting indeed to know the exact cause 
of the obliteration of the teeth of these so-called Margaritanas. 
The teeth of the Naiads seém to be peculiarly susceptible to 
injurious influences, and many cases among them somewhat 
similar to that of the Margaritanas might be cited. In Oris- 
taria, a Chinese and Japanese group, the cardinals are gener- 
ally though not always obsolete, while the laterals in young 
or merely adult shells are developed. Old specimens are fre- 
quently without teeth, like the Anodontas, in which case they 
are probably absorbed in the process of growth. There is a 
group of peculiar Naiads found in the East Indian Archipelago, 
typified by Unio bengalensis Lea, of thin structure and lurid 
purplish or reddish color throughout, having a wide, internal 
prismatic border visible. In all of them the teeth when present 
are greatly compressed, and they occur in various stages from 
a perfect condition to almost complete obliteration, so that the 
species have been divided up between Unio and Anodonta. 
The fact that certain specimens of a given species in the group 
may have well developed teeth, while in others they may be 
almost completely wanting leads me to place all the species, 
which seem to form a very natural group, in Unio. 
Pseudodon is another genus in which it is quite probable the 
teeth have degenerated from some cause until in most cases 
only a single, rounded tubercle, answering to a cardinal, re- 
mains in each valve, and one of the Chinese Naiads Unio 
biasianus is a perfect Margaritana with blurred laterals like M. 
rugosa, though the species probably groups with the well 
known Unio sinensis. And it is likely that Bowrguignat’s 
genus Cameronia, in which the shell is only toothed behind 
the beaks, is a depauperate state of Pleiodon, a genus in which 
the teeth are found throughout the entire length of the hinge 
plate. It is a fact that those species of Unios which seem most 
closely related to the Margaritanas usually have more or less 
imperfect laterals, and sometimes feeble or blurred cardinals. 
In many localities a large proportion of the specimens of 
one or more species of Unio, especially adult or old shells, 
while apparently healthy in every other way show diseased 
hinges in which the epidermis is folded in and greatly pro- 
