STATE GEOLOGIST. 79 



post-abdomen, is usually enlarged, and the anal opening is near its 

 base; the armature is usually considerable. The form of the post- 

 abdomen is one of the best criteria for distinguishing genera and 

 species — a process often attended with much difficuUy. 



The shell is of various forms, frequently beautifully sculptured. 

 The number of eggs produced at onetime is limited, and the winter 

 ^ggs are very often laid in the brood-cavity with no preparation of 

 the shell previous to it, in other words, the ephippium may be ab- 

 sent. On the other hand, sometimes the shell is considerably modi- 

 fied, and generally there is a deposit of dark pigment in the upper 

 part of the shell. The males are very rare and until recently few 

 were known. The diligence of Kurz has added a great many, and 

 we now have a 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-abdo- 

 men. 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 American 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 altera- 

 tions in the form of the post-abdomen consist in a narrowing or 

 excavation of that organ to permit its introduction 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 par- 

 thenogenesis. In some cases it would seem from Weismaon's ob- 

 servations 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 E gives views of typical 

 Lynceidse 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 

 maxillae (not shown) which are exceedingly small, the largely de- 

 veloped 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 developed limb. Fig. 3 shows the appearance of a differ- 



