304 MEMOIRS OF THE CARNEGIE MUSEUM 
October. However, they are not, as a rule, then discharged, but are carried through 
the winter in the marsupium, and are not set free till warmer weather sets in again, 
in April, May, or June of the next year. 
Among the winter-breeders we have in some cases the breeding season still 
more extended, so that in summer (June, July) the end of one season overlaps 
with the beginning of the next (of course, not in the same individual), and in 
such forms gravid females may be found all the year round. | 
In order to understand these conditions, we must inquire what are their es- 
sential features. Sterki used the terms swmmer- and winter-breeders, and he cer- 
tainly was right so far as it concerns the species in our latitude.’ But in my 
opinion, the most important difference of the two groups is, that in the one case 
the breeding season is short, and the glochidia are discharged immediately, while 
in the other case it is long, and the glochidia are kept in the marsupium for a long 
time. This would be well expressed if we call the summer-breeders tachytictic 
forms, and the winter-breeders bradytictic forms.’ It seems to me, that the former 
condition is the primitive and original one, while the latter is apparently a special 
adaptation to the climatic conditions of higher latitudes. From this point of view 
we gain a rather satisfactory interpretation of the raison d’étre of the two types, 
and also ascertain a very significant correlation between these physiological char- 
acters and certain morphological features. 
If a short breeding season is the more primitive condition, we can understand 
perfectly well, that those Najades, which are tachytictic, have also a comparatively 
primitive and simple morphological structure in their marsupium. This is actually 
the case in Margaritana as well as in Quadrula, Pleurobema, and Unio. On the 
other hand, in the forms with a long breeding season we should expect morpho- 
logical adaptations conforming to it. And this is actually the case. In the Ano- 
donta-group we have seen, that there exists a very complex structure of the mar- 
supium, the purpose of which apparently is, to furnish a proper water circulation 
for the embryos enclosed in the marsupium (See above, p. 293), and in all members 
of this group the embryos stay a long time in the marsupium. In the members of 
the Lampsilis-group, which also are bradytictic, the Anodonta-structure is not pres- 
ent. But we have become acquainted above with a peculiar bulging out of the 
marsupium, which apparently serves the same purpose, namely to bring the 
glochidia into closer contact with the water, since the membrane which covers 
this outbulging marsupium is very thin. But in addition, I shall describe below 
‘I know cases of exotic forms, which behave like summer-breeders, but do not breed in ‘‘summer.”’ 
10From tTaxvs quick, Bpad’s slow, and TiKTuKés breeding. 
