188 PROCEEDINGS OF THE NATIONAL MUSEUM vol. io8 



forms, including duplex, crassicutis, denticulatus, and incursus, while 

 remaining smooth and entire in laticollis, retrorsus, hubrichti, and 

 adsinicolus (fig. 4). 



The secondary tibiotarsus is bulkiest near the center of the generic 

 range, where it is also distally modified into a T or Y shape. The 

 marginal forms tend to have a smaller process, with the terminal 

 modification much simpler. In crassicutis crassicutis the subterminal 

 process is rather small and visible only in a sublateral aspect, in 

 crassicutis duplex it is missing entirely. The secondary tibiotarsus of 

 clarus is reduced to a rather short acicular process much smaller than 

 the primary branch. 



Aside from the obvious differences mentioned in the preceding 

 paragraphs, the general shape of the gonopods in the different forms is 

 distinctive for each, embodying numerous elements of shape and 

 proportion difiicult to describe verbally. 



Sternal knobs: In males of the crassicutis group, the podosternites 

 of the midbody segments are produced into transversely elongated 

 swellings between the posterior legpair. Usually two or three of the 

 sternites posterior to the gonopods are flat, with a median cruciform 

 impression, caudad of which the sternal knobs form a continuous 

 series back to the 16th or 17th segment. These swellings are not 

 present in males of P. clarus. Their appearance in the males of some 

 species of Epeloria suggest a line of relationship which is augmented 

 by the presence of a somewhat broadened collum in certain forms of 

 that genus. 



Cyphopod aperture: Females of Pachydesmus appear to be 

 characterized by a specialization affecting the vicinity of the 2d 

 legpair. The anterior margin of the pleurotergite of the 3d segment 

 is produced cephalomesiad into a distinct rounded lobe on each side 

 of the body, these lobes extending partially in front of the coxae of the 

 2d pair of legs. The margin of the pleurotergite immediately posterior 

 to the lobes is raised into a high marginal flange. 



Cyphopods: In the material available to me, females of onl}^ five 

 forms arc represented. In these, however, the shape of the cyphopods 

 is so distinctive that the structures would seem to be readily available 

 for diagnostic purposes. Furthermore, there seems to be structural 

 divergence and similarity on a par with, and correlated with, variation 

 in the gonopods. For instance, the females of the closely related 

 forms laticollis, hubrichti, and retrorsus obviously are quite similar in 

 genital characters (fig. 5,a,c), in that the receptacle (R) is not longer 

 than the valves, and is distinctly obliquely excavated distally, leaving 

 a pronounced shelf just beneath the outer valve. In the female of 

 the more disjunct P. crassicutis duplex, the receptacle (fig. 5,e) is 

 extended distad considerably beyond the ends of the valves, and its 



