204 kobert h. wolcott: 



ropean forms have usually been arranged and with the end in 

 view of bringing allied species together. First the two free- 

 living forms, together with a third, parasitic in habit, but re- 

 sembling the other two very closely in form and structure, fall 

 naturally into what may be called the crassipes group, A. 

 crassipes, A. aculeatus and A. pectinatus. Then the forms in 

 which the hind leg of the male is modified and in which, in 

 the same sex, the body is more or less strongly emarginate 

 posteriorly and which in both sexes have the distal segment of 

 the palpus broad and tipped with two prominent curved claws, 

 form a second group, the intermedins group, including A. 

 intermedins, A. abnoiwiipes, A. indistinctus and A. serratus. 

 A third group includes A. fossulatus and A. strieta, with 5 

 acetabula on each side, in the female set into the surface of the 

 body, in the male on a genital plate; while A. arcuatj, A. 

 yjpsilophorus and A. tuviidus form a not very homogeneous 

 group, the members of which are related in form of body, 

 character of genital area, etc., and which in structure and po- 

 sition of the genital area show a gradual transition towards A. 

 (iV.) ingens, which itself is related on the other hand to 

 Cochleophorus. 



In the descriptions which follow, the terms used need no 

 explanation except perhaps with reference to the palpi and legs ; 

 the former are supposed to be extended parallel to the long 

 axis of the body, whence the terms inner and outer, dorsal, ex- 

 tensor or upper, and ventral, flexor or lower, as applied to the 

 corresponding surfaces. The legs are supposed to be at right 

 angles to the body and the terms anterior and posterior become 

 applicable. The names applied to various parts are such as 

 have hitherto been applied or are literally translated from the 

 corresponding terms in use by European writers on the subject. 



Measurements have been taken with an ocular micrometer. 

 The length of the body is that of the body proper, and the 

 projecting mouth-parts are not included; the length of a seg- 

 ment is the length of a straight line connecting the middle 

 points of its two ends; and the total length of an appendage is 

 the sum of such lengths of the segments which compose it. In 



