ZOOGEOGRAPHY 371 



The Central American subregion includes Salvador, from which no psel- 

 aphids are known. The northern part of Mexico is nearctic, as is also an inland 

 extension southwards involving the Rocky Mountain-Andean Cordillera. This 

 part of Mexico has been subtracted to give the 600,000 square miles for the sub- 

 region. In general terms, from the Tropic of Cancer to South America. This area 

 holds many endemic genera and is exposed to dispersal from the South American 

 subregion on the south and possibly from the Nearctic Region on the north. This 

 is to be discussed in detail shortly. 



The Antillean subregion now consists of a chain of islands which begins 

 near the mouth of the Orinoco River and extends northwestward nearly to Flor- 

 ida. Therefore it is discontinuous and its insular units collectively have consider- 

 able variation in local weather and terrain. The taxonomic density of 1 : 1476 

 is superficially impressive. Actually the pselaphid fauna is poorly known. Two 

 local studies would be of interest: a comparative analysis of Trinidad and the 

 Orinoco delta; a similar study of the pselaphids of three areas, namely extreme 

 southern Florida, northwestern Cuba, and eastern Yucatan. From purely faunal 

 needs, specimens from Jamaica are greatly to be desired. The figure of 90,000 

 square miles includes Jamaica, Trinidad, Barbados, and the Bahamas from 

 which no pselaphids are known. 



The third is the South American Subregion, with a density of 1 : 10,138 

 for the whole. The figure of 6,600,000 is arrived at by subtracting the southern 

 half of Argentina and southern Chile. At this early stage there appear to be at 

 least two centers of speciation. The Matto Grosso area of southwestern Brazil, 

 eastern Bolivia, and northern Paraguay; the upper reaches of the Amazon in 

 northwestern Brazil, southern Colombia, and southern Venezuela. 



The three subregions having been outlined, it is possible to examine the 

 affinities of Barro Colorado Island. This small locality is well known in Psel- 

 aphidae, and such an analysis is profitable since the Panamanian Isthmus sepa- 

 rates two of the three subregions. 



On this island there are thirty-one genera, and forty-four or possibly 

 forty-five, species. Of these genera, eleven are found nowhere else; of the 

 species, only two are found elsewhere. This high degree of endemism is un- 

 doubtedly a consequence of lack of information in large measure. It is not 

 even a peculiar situation in neotropical pselaphids, as we know them. That 

 is, an area thoroughly worked will present the same picture, whether it be in 

 Mexico, or Brazil. 



Taking the situation on the island as we find it today, the genera Sebaga, 

 Jubus, Eurhexius, Eupsenina, Xybarida, Xybaris, Arthmius, Oxarthrius, 

 Euphalepsus, Batrybraxis, Neotyrus, and Hamotus are predominantly South 

 American. 



The genera Thesium, Trimiopsis, Scalenarthrus, Drasinus, Dahnoburis 

 predominantly Central American — not taking into account the eleven endemic 

 genera, which may prove to be either northern or southern in distribution. 



The genus Berdura is Antillean. 



The genus Reichenbachia is cosmopolitan and the genus Decarthron al- 



