32 NAIADES OI? MISSOURI 



Jor 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 

 Missouri 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 (vSay) and costata (Raf) [ = undidatus (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 



