l88 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [Bull. 



also two pairs of small supplementary ones on the succeeding 

 segments. In the genus Tanais the oostegites are limited to a 

 single pair borne on the fifth segment. 



The abdomen consists typically of six segments, as is clearly 

 indicated by the appendages ; the five anterior segments are 

 short, the terminal one is large. The abdominal segments suffer 

 various reductions in number by fusion with each other. Thus 

 in the Aselloidea, and the genera Erichsonella, Jcera, and Edotea 

 all the segments are united to 1 a single large plate ; in the Spsero- 

 midae and females of Cyathura the five anterior abdominal seg- 

 ments are fused together; in Idothea the abdomen consists of 

 two short segments and a large terminal one; in Chiridotea, of 

 three short ones and a large terminal one ; and in Tanais, of four 

 short ones and a large terminal one. 



The appendages of the abdomen consist of pleopods and uro- 

 pods, but unlike the Amphipoda there is never more than one pair 

 of uropods, which are terminal in position in the Tanaioidea, the 

 Oniscoidea, and Aselloidea, and lateral in the Cymothoidea and 

 Idotheoidea. In the Tanaioidea the uropods are composed of 

 many joints but in the Oniscoidea and Aselloidea the rami are not 

 jointed but styliform. In the Cymothoidea the uropods are more 

 or less lamellar in form, consisting of a peduncle and two oval 

 rami, and with the terminal segment of the abdomen, with which 

 they articulate laterally, form a kind of caudal fan somewhat 

 like that of the Macrura. In the Anthuridse the outer ramus 

 occupies a superior position arching over the telson, and in the 

 Sphseromidse the inner ramus is fixed and only the outer one is 

 movable. In the Idotheoidea the uropods close over the pleopods, 

 meeting ventrally in the middle line like double doors. These 

 valves articulate with the terminal segment of the abdomen only 

 'along the lateral margin. 



The pleopods are usually present in five pairs and consist of 

 a peduncle and two lamellar rami. They function for swimming 

 and respiration; and in the male the inner ramus of the second 

 pair bears a slender stylet which aids in copulation. In the 

 Cymothoidea the pleopods are both natatory and respiratory; 

 in the Aselloidea and Idotheoidea they are respiratory exclusively ; 

 and in the Tanaioidea they are exclusively natatory, respiration 

 in this group being accomplished by means of special chambers 



