ORTMANN: A MONOGRAPH OF THE NAJADES OF PENNSYLVANIA 303 
important differences between the different forms with regard to the time, when 
the marsupium is filled, and the present writer has devoted a special article to 
this topic as far as it concerns the Pennsylvanian species (Nautilus, 1909), which 
confirms and supplements Sterki’s observations. Additional facts have been fur- 
nished by Conner (1909, p. 112). 
Since the publication of my paper in 1909, I have made a large number of 
further observations, which generally serve to support my previous conclusions, 
and the microscopical investigation of the contents of the marsupia of soft parts 
at hand has added a number of more important facts. These new observations 
will be reserved for the systematic part, and will be treated under each species. 
Here I shall give only a condensed statement of the results obtained by Sterki 
and myself, and a few considerations as to their systematic value. 
Among our Najades, we have in our latitude two types, swmmer-breeders and 
winter-breeders. ‘Those belonging to the first class, have the marsupium filled only 
in summer, that is to say, in the period between the end of April and the beginning 
of August. Of course this indicates the extremes; for in the case of single species 
and single individuals the breeding season is much shorter; and the month of 
June, and the beginning of July is generally the time when most of these forms are 
gravid. The development from the egg to the glochidium takes place within a 
very short time (a few weeks), and, after the glochidia are formed, they are dis- 
charged immediately. The consequence is that individuals with glochidia are 
rather rarely found. 
It is a remarkable fact, first pointed out by Sterki, that summer-breeders 
are found only in a restricted group of genera; and, as we shall see, this physio- 
logical character is always found in the forms belonging to this group. It is com- 
posed of the genera: Quadrula, Pleurobema, and Unio of Simpson, and, as may 
be seen from the description of their soft parts given above, these, according to 
their morphological characters, form a group by themselves. Likewise the genus 
Margaritana, which stands by itself in the system, seems to be a summer-breeder; 
at all events, Conner (1909, p. 112) found gravid females in June and August, 
but not in November and January, and I failed to find them in the beginning of 
May, the beginning of June, and on August 4. (At the latter date only one speci- 
men was taken.) In Europe, Margaritana is known to breed in July and August 
(See Harms, 1907, p. 818). 
All other genera are winter-breeders. In these the marsupia fill some time 
in summer, from July to September. The development of the eggs also takes 
place rapidly, and generally the glochidia are fully developed in September or 
