THE NAUTILUS. 91 
“SOME NOTES ON THE GENITAL ORGANS OF UNIONIDE, WITH REFER- 
ENCE TO SYSTEMATICS. 
BY DR. VY. STERKI. 
It is generally known that in a group of Uniones comprising a 
great part of our species, such as ligamentinus Lam., rectus Lam., 
subovatus Lea, parvus Barnes, nasutus Say, rangianus Lea, alatus 
Say, ete., the females have the posterior part of their outer branchie 
peculiarly transformed into “branchial uteri,” and also that in con- 
-sequence of this, in almost all instances the shells of the female 
specimens are distinctly and decidedly dilated downward at the 
posterior end, so as to be distinguished from the males at first glance. 
The degree of difference between the sexes is, however, very differ- 
-ent among the several species, but it is constant. For convenient 
reference in the following, this group is designated as A. There is 
another group, say B, in which all four branchie are charged in 
their totality with embryones, as already shown by Lea for some 
-species,’ but do not show such marked transformation and change 
in colors as those of the former group, and cause also no such strik- 
ing differences in the shape of the mussel, according to the sexes. 
Examples of this group are: U. subrotundus Lea, pustulosus Lea, 
aesopus Green, undulatus Barnes. 
There are some facts of peculiar interest in connection with this 
grouping. The first is that the animals are propagating at certain 
seasons quite different for the two groups, as the writer has ascer- 
tained by examining thousands of specimens during the last four or 
five years. In group A the branchial uteri are charged with embry- 
-ones from late summer to the beginning of winter, and probably in 
most through the winter, while in early summer they are empty, the 
-embryones having been discharged. At that time the ovaries of the 
females are charged with ova, and the testes of the males with sper- 
matozoa, while the latter are missing, or quite scarce, in the time 
from late summer to winter, in which time the embryones attain 
their maturity in the branchiz. 
In group B, just the reverse is true. During the fall, 7.e., about 
from August, and probably winter, the branchiz are empty, con- 
‘taining no embryones, while the ovaries are filled with ova, and the 
'Yet Huxley, in his valuable “Man. Anat. of Inv. An.,” says: “In Unio 
-and Anodonta the young are hatched in the outer gill pouches of the parent 
”? 
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