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 Pleuroxus 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 (Chijdonis) 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 

 Cladocem, 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 {Eurycercus alone excepted). The males 

 are usually but not always smaller. Plate LXI gives views of typical 

 Lynceidm. Fig. 1 is particularly instructive, tor 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 antennulesCA^), the keel of the labrum (Lb.), 

 the peculiar modification of the first pair of feet to assist the maxillae 

 (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- 

 oped limb. Fig. 3 shows the appearance of a different embryo from be- 

 lowand in an earlier stage. Fig. 2 illustrates the 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 EURYCERCIN^. 



A single species constitutes the sub-family, and it will be necessary 

 to point out only those points which are distinctive. 



The Earycercince differ from the true Lynceidce and approach the 

 LyncodapJmidce in having the digestive tract not coiled, with two caeca 



