114 THE AMERICAN MIDLAND NATURALIST 
for North Missouri under the so-called plicata (Say) types, for the 
most part; those of Central Missouri under both types of plicata 
and: the better known undulata (Barnes), and those of Southern 
Missourt under undulata. These facts might be accounted for 
by the natural physiological adjustment to ecological conditions— 
that is to say, the quiet, sluggish, muddy streams of North Missouri 
tend to produce a heavy, inflated, rarely plicated shell, mostly 
represented by Amb. rariplicata of Deshayes; on the other hand, 
the swift, clear water streams of South Missouri have the tendency 
to shape up a compressed and multi-plicated shell best represented 
by Amb. perplicata quintardi of Cragin, while the intermediate 
or combined ecological conditions of Central Missouri give combi- 
nations of these two extremes. In the grouping of the members 
of this genus there has been much necessary elimination of local 
varieties and races and thus types have been adherred to as much 
as possible. The arrangement is only submitted as tentative due 
to the doubt of the present nomenclatural situation. This problem 
may be easily solved if it may be found that the morphology of 
shell characters may be traced, in most instances, to ecology. Prob- 
ably this solution may be accomplished by studies of closely con- 
nected series from the glochidial to the mature shell. Our judgment, 
from studies of local conditions in this state, would be that the 
obliquely undulated and plicated folds, forming the chief shell 
character of this genus, are more developed in swifter current 
as a physical adaptation for survival by the way of more permanent 
anchorage, etc., just as we may account for the pustulate and 
nodulous characters of the shell instead of considering them as 
mere characteristic markings. However, when it has been found 
that the beak sculpture (the most constant shell character) of Amb. 
plicata (Say) and costata (Raf) [=undulatus (Barnes)| are really 
different and that there has been a differentiation from the adole- 
scent shells to the mature ones we are compelled to recognize 
genetic distinctions in these two species. Yet it seems that it may 
be safely stated that two such well defined groups are connected 
in all manner of inter-grades through environmental causes such 
as seen in the different ecological provinces of Missouri. It is 
found that this genus has a short period breeding season, that the 
white, leaf-shaped conglutinates are discharged by the natural 
outlet of the anal opening and that these are delivered in broken, 
loose masses just as soon as the larvae are mature, or even ejected 
