STATE GEOLOGIST. 135 



the shorter branch bearing seven setai, of the other four, the proxi- 

 mal joint of the former with three stout spines; the maxillse nearly- 

 like Diaptomus; the processes have respectively the following 

 numbers of setse: the basal plate eight, the small processes at base 

 of posterior branchial appendage one, the appendage itself twelve, 

 terminal portion three groups, first containing nine, the second 

 three, and the third four or five, the upper of the anterior processes 

 two, and the lower three; fifth feet nearly like the others in size; 

 the right in the male having the outer branch but two-jointed by 

 the coalescence of the two outer to form an arcuate and deformed 

 appendage, armed at the end with three stout equal spines; corres- 

 ponding branch of left foot three-jointed; the terminal joint bearing 

 three unequal spines, each of the preceding joints only one; inner 

 branches similar, three-jointed; terminal joint being short and armed 

 with three short lanceolate setas and three longer ones, two of which 

 are curved so as to be slightly prehensile; fifth foot of female with 

 both rami three-jointed; inner ramus much smaller; antepenult 

 segment of the outer ramus extending into a large lanceolate pro- 

 cess; ova-sac long-ellipsoidal or spherical, reaching nearly to the end 

 of the caudal setse." 



V. — Genus Diaptomus, Westwood. 



The most widely distributed and well-known of fresh water Cala- 

 nidse, inhabiting in various species the smallest as well as the largest 

 bodies of standing or sluggishly-flowing fresh water. Apparently 

 a recently formed group whose nearest known ally is the curious 

 Pseudo-diaptomus, found in the gulf of Mexico. The animals of 

 this genus are apparently very susceptible to the influences of the 

 environment, and are consequently extremely variable not only in 

 color but in minor structural points. In America there is a curious 

 fact, which is susceptible of different explanations, one of which 

 was given in the American Naturalist at various times during the 

 year past. The species or varieties fall in pairs, one of which is 

 smaller and less highly differentiated, while the other is greatly en- 

 larged and has the peculiarities emphasized. These sets occur in 

 open and shallow water respectively. The large varieties are, as 

 the rule, restricted t© such shallow weedy pools as dry up during 

 summer and freeze solid in winter. The forms intermingle slightly, 

 but there are seasonal differences of greater or less extent. 



The body is composed of an elongated thorax, with which the 

 head is united, forming a six-jointed cephalothorax. The abdomen 



