MILLIPED GENUS DIXIORIA — HOFFMAN 19 



between these large, relatively stable, populations, in the few known 

 instances, occurs in narrow belts between them. 



A diagram (fig. 3) indicates the relationships of the subspecies of 

 pela, of which lines of affinity are obvious from both geographical and 

 structural considerations. I have omitted only an indication of the 

 direction of evolution. That it has been linear is indicated by the 

 progressive nature of the changes and by the fact that the most modi- 

 fied form {wrighti) finds itself juxtaposed geographically with the 

 simplest (pela). I believe that this militates effectivel}^ against the 

 possibility of simultaneous, in situ evolution from a widespread parent 

 stock. 



The geographical evidence suggests a northward spread through 

 the Iron Mountains, the encounter of a physiographic barrier (the 

 reduction of the uplands by the New River and Tennessee River head- 

 waters) and subsequent exploration back to the south on a subparallel 

 but isolated mountain range. The presence of Dixioria pela fowleri 

 on Big Walker Mountain in Bland County, Va., indicates that its 

 isolation from the bulk of the Dixioria population to the southeast 

 must have been relatively recent, perhaps by acceleration of base- 

 leveling by a local uplifting during the Quarternary. 



Literature cited 



Causey, Nell B. 



1942. Six new diplopods of the family Xystodesmidae. Ent. News, vol. 53, 

 pp. 165-170, 9 figs. 

 Chamberlin, Ralph V. 



1918. New polydesmoid diplopods from Tennessee and Mississippi. Psyche, 



vol. 25, No. 6, pp. 122-127. 

 1947. Some records and descriptions of diplopods chiefly in the collection of 

 the Academy. Proc. Acad. Nat. Sci. Philadelphia, vol. 99, pp. 

 21- 58, 73 figs. 

 Hoffman, Richard L. 



1949. Nine new xystodesmid millipeds from Virginia and West Virginia, 

 with records of established species. Proc. U. S. Nat. Mus., vol. 99, 

 pp. 371-389, 18 figs. 



U. S. GOVERNMENT PRINTING OFFICE: 1953 



