ENTOMOSTRACA OF MINNESOTA. 225 
fair idea of the sexual variations. These consist usually in a narrower 
body and shorter beak, in a strong hook of chitin on the first foot and 
certain modifications of the post-abdomen. The hook mentioned is 
simply an enlargement of one of the terminal bristles of the foot, and 
serves to fasten the animal to the shell of the female. In one Ameri- 
can species of Plewroxus we find an approach to this structure in the 
female—an interesting example of inheritance of sexual peculiarities 
across the sexes. The alterations in the form of the post-abdomen 
consist in a narrowing or excavation of that organ to permit its intro- 
duction into the brood-cavity, and in some forms (Chydorus) this 
change can only be understood by observing the form of the shell of 
the female about to produce winter eggs. In general, as in other 
Cladocera, males are found only at the period when the females are 
sexually perfect. The ordinary method of reproduction is by virgin- 
bearing or parthenogenesis. In some cases it would seem, from Weis- 
mann’s observations, that the sexual method occurs only incidentally. 
The orifice of the male organs is between, or anterior to, the terminal 
claws of the post-abdomen (Hurycercus alone excepted). The males 
are usually but not always smaller. Plate LXI gives views of typical 
Iynceide. Fig. 1 is particularly instructive, for in it the details 
which can be usually made out in the living object are represented. 
The following points may be especially noticed. The large size of the 
pigment fleck, the large antennules (A?), the keel of the labrum (Lb.), 
the peculiar modification of the first pair of feet to assist the maxille 
(not shown) which are exceedingly small, the largely developed anal 
gland (A. g.), the form and muscular mechanism of the abdomen, 
which, however, is better illustrated by Fig. 10 of the same plate. 
Fig. 1 contains an embryo seen from the side with the partially devel- 
opedlimb. Fig. 3shows the appearance of a different embryo from be- 
lowand in anearlierstage. Fig. 2 illustratesthe relation of the brain 
to the eye and the very small optic ganglion. Fig. 9 of Plate LXIII 
gives details of the feet in another species, and the modifications seen 
in the male of the same species are sufficiently shown in Fig. 1 of the 
same plate, which also well illustrates the various sculpture of the 
shell displayed by this group. Figs. 4 and 9 of Plate LXIV show 
curious modifications of the post-abdomen of the male, and Fig. 7 ex- 
hibits the structural peculiarity of sexually perfect females which is 
correlated with it, or, perhaps, we may say, occasions it. 
SUB-FAMILY EURYCERCINE. 
A single species constitutes the sub-family, and it will be necessary 
to point out only those points which are distinctive. 
The ELurycercine differ from the true Lynceide and approach the 
Lyncodaphnide in having the digestive tract not coiled, with two cec, 
