88 TWELFTH ANNUAL REPORT. 



(b) Head depressed, acute ; post-abdomen excised near the anus. 



3. Genus Crepidocerctis. 



C. Post-abdomen more or less quadrangular, armed with one or two rows of small 

 teeth on either side behind ; terminal claws with one or two basal spines ; hight of 

 posterior shell margin usually less than the greatest hight of shell. 



4. Genus Lynceus. 



D. Greatest hight of shell little less than that of posterior shell margin ; post-abdo- 

 men terete ; terminal claws very minute. 



5. Genus Phrixura. 



E. Greatest hight of shell more than double that of posterior margin. 



(a) Eye and first foot normal. 



6. Genus Chydorus. 



(b) First foot with a claw which extends beyond the shell. 



7. Genus Anchistropus. 



(c) Eye absent, only pigment fleck used for vision. 



8. Genus Monospilus. 



IV. — Genus Leydigia, Kurz. 



In this genus, both the known species of which are found in 

 America, the posterior part of the shell and body is emphasized at 

 the expense of the anterior. The curved posterior margin is equal 

 to the greatest hight of the shell. The head and anterior part of 

 the body are of the form characteristic of Alona; indeed, the whole 

 body is in plan like Alona, but in the back part the organs are all 

 enlarged. The general form of the body and abdomen recalls 

 Ilyocryptus; the post-abdomen, in particular is very like that genus. 

 The last two pairs of feet are much enlarged. The shell is usually 

 irregularly marked with longitudinal striae; the lower margin 

 is covered with long spine-like setae. The post-abdomen is armed 

 with several sets of long spines and aggregations of bristles and 

 small spines; it is almost round and enormously enlarged. The 

 intestine is coiled and expanded at the end, but the anal caecum is 

 rudimentary. The antennae are heavily spined and have eight setae; 

 the labrum is more or less hairy. The male has a strong hook on 

 the first foot, and between the terminal claws of the abdomen is a 

 peculiar intromittent organ. 



Sp. 1. LiCydigia quadrang-ularis, Leydig. 



(Plate H. Fig. 4.) 



Lynceuit quadrangularis, i.^Yi>iG,FRic. 

 A.lona leydigii, schoedler, p. e. mueller. 

 Leydigia quadrangularis, kurz. 



The shell is comparable to that of Alona quadrangularis, but 

 higher behind; the markings are not very distinct; shell trans- 

 parent. The head is very small; the eye smaller than or of about 

 the size of the pigment fleck. The post-abdomen is very broad, the 



