198 PROCEEDINGS OF THE NATIONAL MUSEUM vol.108 



center of the range occurs a phi-atry of four closely related sub- 

 species: laticollis, retrorsus, hubrichti, and adsinicolus. These are 

 similar in their large size, testaceous color pattern, and in several 

 particulars of the gonopods. Around the periphery occur four other 

 subspecies: crassicutis, duplex, denticulatus , and incursus, all of which 

 are smaller, more colorful, and similar in gonopod structure, sharing, 

 for instance, a serrated tibiotarsal blade. These marginal forms 

 may be considered representatives of the ancestral stock, now more 

 or less isolated by the centripetal evolution producing the larger, 

 more specialized, interior forms. 



It is interesting to note that in the several races of which female 

 specimens are available, the cyphopods show differences and similari- 

 ties in almost the same magnitude as do the gonopods of the cor- 

 responding males. 



Key to the subspecies of Pachydesmus crassicutis, based on males 



1 . Pref emur of gonopod shorter, 60 percent or less the total length of the telopodite 



joint; mesial edge of tibiotarsal blade serrate or dentate (fig. 7, a-d) ; secondary 

 tibiotarsus usually straight and subparallel to the main branch, its subtermi- 

 nal tooth generally much reduced or absent, if present usually directed distad, 

 making the end of the process Y-shaped; animals smaller, adult generally 



less than 65 mm. in length {crassicutis phratry) 2 



Pref emur longer, 65-70 percent of the total length of the telopodite; mesial 

 edge of tibiotarsal blade smooth; secondary tibiotarsus usually sinuate and 

 divergent from the main branch, its subterminal tooth well developed and 

 usually retrorse, directed proximad, making the end of the process T-shaped; 

 animals larger, adults generally more than 65 mm. in length {laticollis 

 phratry) 5 



2. Subterminal tooth of secondary tibiotarsus completely absent; length, 59 to 



61 mm. (northern Mississippi) crassicutis duplex Charaberlin 



Subterminal tooth of secondary tibiotarsus present but variable in form, 

 occasionally quite small 3 



3. Secondary tibiotarsus distinctly longer than primary, its subterminal tooth 



reduced to a small subtriangular lobe or spur; length, 51 to 65 mm. (western 



South Carolina, northern Georgia) crassicutis incursus ChamberUn 



Secondary tibiotarsus not longer than primary, its subterminal tooth larger, 

 in the form of a definite projection 4 



4. Secondary tibiotarsus slightly sinuous, its subterminal tooth perpendicular to 



the main axis; coxal apophysis elongated, distally rounded with the margin 

 entire; length, 50 to 61 mm. (north-central Georgia). 



crassicutis denticulatus Chamberlin 

 Secondary tibiotarsus almost straight, its subterminal tooth directed distad; 

 coxal apophysis short, its distal edge distinctly indented; length, 53 to 63 

 mm. (southern Mississippi, eastern Louisiana). 



crassicutis crassicutis (Wood) 



5. Tibiotarsus of gonopod shorter, bulkier, more triangular distally, only 30 per- 



cent of the length of telopodite 6 



Tibiotarsus of gonopod longer, more slender, less enlarged distally, 35 percent 

 of the length of telopodite 7 



