216 PROCEEDINGS OF THE NATIONAL MUSEUM vol. los 



he did not mention the basis for his opinion. I suspect his associ- 

 ation must have been founded upon species really not congeneric with 

 Rhysodesmus Umax and Pachydesmus crassicutis, although lumped 

 with them in his rather un wieldly classification. 



In my own opinion, the existing genus most closely allied to Pachy- 

 desmus is Epeloria (Chamberlin, 1939), at least the species related to 

 E.jicta, E. dela, and E. leiacantha. In this genus the podosterna of the 

 males are produced into distinct transverse ridges, and the collum is 

 definitely broader than the following tergite. The gonopods appear 

 bifid near their ends, an effect readily achieved by the basal coalescence 

 of the primary and secondary tibiotarsi. From a generalized ancestral 

 stock, perhaps similar to P. clarus, it is quite feasible to derive two 

 specialized groups, one represented by the large races of P. crassicutis, 

 the other by Epeloria talapoosa and its relatives, which have remained 

 small and developed a simplified gonopod but which are otherwise 

 rather similar in body form and coloration to P. crassicutis. Their 

 distribution around the southeastern periphery of the crassicutis 

 group adds a certain amount of credence to the possibility of a com- 

 mon origin. For the present it is sufficient to indicate that if the 

 telopodite of the gonopod of Epeloria ficta, for example, was bifid 

 entirely to the base instead of only halfway, the result would be almost 

 identical to the gonopod of P. clarus, and the similarity of body 

 structure is so close that it need scarcely be changed to approximate 

 that of a true Pachydesmus. 



Specific relationships: As already implied, P. clarus seems to 

 represent clearly the ancestral stock from which P. crassicutis and its 

 subspecies evolved. It presents none of the specializations peculiar 

 to the larger species, such as more complex gonopods, increase in 

 size, broadened collum, and podosternal ridges of the male. Specific 

 evolution has perhaps been accelerated and almost certainly facili- 

 tated by the Mississippi flood plain, which so far as now known com- 

 pletely isolates P. clarus in the hill parishes of western Louisiana. 



The eight recognized subspecies of P. crassicutis fall into two dis- 

 tinct ensembles which have been defined above. Of these two 

 phratries, that including crassicutis, duplex, denticulatus, and incursus 

 is the more primitive. The species are smaller, with less specialized 

 genitalia, and occupy peripheral locations on the range of the genus. 



During the Cretaceous embayment of the Coastal Plain province, 

 animal life which had occupied that region must have been forced to 

 withdraw to inland areas. Subsequent invasion of the emerged 

 coastal plain would provide the opportunity for the ancestral pachy- 

 desmid stock to spread southeastward probably from Ozarkian and 

 Appalachian refuges. The conservative elements remained west of 

 the Mississippi in the form of P. clarus, the more aggressive produced 



