224 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



FAMILY LYJSCEIDJ]. 



This family, which is numerically the largest among the Cladocera, 

 is, in the main, well limited, though there are transitions toward the 

 Lijncodaplinklce^ which are quite direct. The genera Lyncodaphnia, 

 Ofryoxus and Ilyocryptus lead toward the Lynceklce unmistakably. 

 Most of the members of this family are small, comparatively few ex. 

 ceeding one millimeter in length. The head is covered with an arched 

 shield, which frequently passes with no indentation into the shell of 

 the body. This head covering generally extends forward and down- 

 ward to form more or less of a sharp angle in front, while in several 

 genera it is simply rounded in front. It, in either case, arches over 

 the more fleshy lower side of the head from which hang the two short 

 antennules and the labrum, while the strong two-branched antennte 

 spring from well up under its posterior expansion. The rounded sides 

 of this shield, which protect the insertion of the antennae, are called 

 the fornices. Above the insertion of antennules is a dark fleck lying 

 near or on the lower angle'of the brain; this is the larval or nauplius 

 eye, w^hich is the first to appear in all these small Crustacea. This 

 macula nigra is not infrequently as large as the eye itself,* or even 

 larger, and in one genus it is the only visual organ. The antennules 

 are small and bear on the end several sensory filaments as well as a 

 lateral flagellum. The antennules of the male dififer very little from 

 those of the female. The labrum is furnished with a process, which 

 is triangular or semicircular and is usually larger than the terminal 

 portion. The mandibles are as in Daplinidce but usually shorter. 

 Maxillse are often conspicuous, but the first pair of feet serve, by a 

 slight alteration at the base, the same purpose. There is rarely an 

 indication of the sixth pair of feet, and the antennse have both rami 

 three-jointed. The terminal part of the body, or post-abdomen, is 

 us'ially 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 difficulty. 



The shell is of various forms, frequently beautifully sculptured. 

 The number of eggs produced at one time is limited, and the Avinter 

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

 the shell previous to it; in other words, the ephippium may be absent. 

 On the other hand, sometimes the shell is considerably modified, and 

 generally there is a deposit of dark pigment in the upper i^art 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 



* The name "Lynceus " is derived from that of the son of Aphareus who was famous for the sharp- 

 nameness of his yision. 



