THE NAUTILUS. 91 



SOME NOTES ON THE GENITAL ORGANS OF UNIONIDJE, WITH REFER- 

 ENCE TO SYSTEMATICS. 



BY DR. V. STERKI. 



It is generally known that in a group of Uniones comprising a 

 great part of our species, such as ligamentinua Lam., rectus Lam., 

 subovatus Lea, parvus Barnes, itasutus Say, raiigianus Lea, alatw 

 Say, etc., the females have the posterior part of their outer branchiae 

 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 branchial are charged in 

 their totality with embryones, as already shown by Lea for some 

 species, 1 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. subrotundu* 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 branchiae. 



In group B, just the reverse is true. During the fall, i.e., about 

 from August, and probably winter, the branchiie are empty, con- 

 taining no embryones, while the ovaries are filled with ova, and the 



'Yet Huxley, in his valuable "Man. Anat. of luv. An.," says: "In Unio 

 and Anodonta the young are hatched in the outer gill pouches of the parent 



