18 PROCEEDINGS OF THE NATIONAL MUSEUM vol. hi 



admittedly preliminary studies — serves a useful piu-pose in con- 

 tributing to the taxonomic stability of the family, and now sufficient 

 material has accumulated to justify yet another. 



Most of the material upon which the following discussion is based 

 is from the collection of the U.S. National Museum (USNM), made 

 available to me through the kindness of Ralph E. Crabill and 

 J, F. Gates Clarke. James A. G. Rehn kindly permitted me to study the 

 holotype of Platyrhacus atratus at the Academy of Natural Sciences 

 of Philadelphia (ANSP). My friend R. W. Lichtwardt of the 

 Botany Department, University of Kansas, is due my heartiest 

 thanks for his gift of the type specimen of the new Nyssodesmus 

 herein described. 



The existence of validated but unidentifiable names in the literature 

 is a matter of much concern to systematists, and it is always gratifying 

 to be able to dispose of such enigmas. Inasmuch as both genera and 

 species of diplopods are based largely on male genitalia, and since 

 this fact was not fully appreciated by most early workers, present- 

 day students of the group find themselves with more than theu' share 

 of distressing and often insoluble taxonomic problems. For example, 

 14 generic names have been proposed for American platjo-hacids, 

 including the typical genus of the family. The type species of only 

 three of these genera were illustrated by their authors, and even 

 these drawings were far from satisfactory. The resulting confusion 

 can be imagined, particularly since most species of Diplopoda are 

 still known only from the type specimens. Some workers dealing 

 with platyrhacids made a bold eflFort to mterpret the subtle (or 

 meaningless) original diagnoses; some selected names arbitrarily; 

 others gave up in despair and lumped all of the Neotropical species 

 into one big catch-all genus; and at least one disdained the ancient 

 tangle and merely proposed new generic names whenever the occasion 

 seemed to require. 



The difficulties that we have here are great ; however, some progress 

 is slowly being made. The following list includes the American 

 generic names that have to be considered, with an indication of theu- 

 current status: 



Platyrhacus Koch (1847): Valid 

 Cyphorrhacus Cook (1896b): Unknown 

 Psammodesmus Cook (1896b): Valid 

 Barydesmus Cook (1896b): = Platyrhacus 

 Nyssodesmus Cook (1896b): Valid 

 Rhyphodesmus Cook (1896b): Valid 

 Nanorrhacus Cook (1896b): Valid 

 Tirodesmus Cook (1896b): ?Valid 

 Spilodesmus Cook (1896b): Unknown 

 Arcydesmus Cook (1896b): Unknown 



