20 PROCEEDINGS OF THE NATIONAL MUSEUM vol. in 



Platyrhacus is a mechanical adaptation to bring the smaller and more 

 delicate solenomerite into a position adjacent to the sternum of 

 segment 6 where the solenomerite is shielded, in situ, by the larger 

 and broader tibiotarsal element. In Psammodesmus , where the 

 solenomerite is already on the inner edge of the telopodite, there is 

 less necessit}^ for such modification. 



Divison of the Platyrhacidae into subfamilies has been essayed 

 but once, in the very brief conspectus of Oriental forms by O. F. Cook 

 (1896a). His subfamilies were based largely upon characters of 

 body form and ornamentation, and were never recognized by later 

 workers. Although a classification of the entire family on the basis 

 of gonopod structure remains to be accomplished, it does not seem 

 premature to recognize the affinities of various genera, when rela- 

 tionships are evident, by the proposal of tribal names, some of which 

 may subsequently be elevated to the rank of subfamilies. A start 

 in this direction is made with the two groupings of genera proposed 

 in this paper. 



Platyrhacini, new tribe 



Composed of platyrhacid genera in which the gonopods are simple 

 in form, the prefemur and femur forming a straight or nearly straight 

 trunk at right angles to the coxite and without special processes; 

 tibiotarsus represented by a thin laminate process directed or bent in 

 the direction of the coxal axis, usually more or less twisted or bent 

 slightly mesiad and shielding a small short solenomerite that originates 

 from the edge of the telopodite away from the coxa. Seminal groove 

 proceeding distad from its origin along the coxal side of the telopodite, 

 thence crossing its mesial face to gain entry to the solenomerite. 



This tribe includes four American genera, separable by the char- 

 acters stipulated in the following key: 



1. Tibiotarsus of gonopod directed distad in line vxith the median axis of tlie 



femur Tirodesmus Cook 



Tibiotarsus of gonopod distinctly turned cephalomesiad at an angle of 30 to 90 

 degrees from the median axis of the femur, in the direction of the coxa . . 2 



2. Lateral margins of paranota set off as distinct smooth rims, not produced into 



dentations; hypoproct rounded in outline . . Aymarcsmus Chamberlin 



Lateral margins of paranota not set off, the upper surface continuous to the 



edge, v;hich is usually provided with marginal dentations or large tuber- 



cules; hypoproct subtrapezoidal in outline 3 



3. Distal third of telopodite bent toward the coxa at an angle of about 90 



degrees Nyssodesmus Cook 



Distal third of telepodite bent coxad at a much smaller angle, normally less 

 than 45 degrees Platyrhacus Koch 



In addition to the gonopod character cited, Tirodesmus differs from 

 the other genera by a peculiar formation of the paranota that may be 

 only specific in value, yet has not been observed in any of the other 



