50 COOK 



posure to unfavorable conditions, they have been unable to extend them- 

 selves in other than the most gradual manner, so that identity, or even 

 similarity, of diplopod faunas affords practically indubitable evidence, 

 not only of land connection, but of existence of continuously favorable 

 conditions, both in space and in time. On this account a comparison 

 of the Diplopoda of northwestern America with those of northeastern 

 Asia may yield valuable evidence regarding the nature and extent of 

 the land connection supposed by many naturalists to have existed for- 

 merly. The Diplopoda of the Harriman Expedition are thus an initial 

 contribution to this phase of the trans-Bering question, though their 

 significance can not be estimated until the Siberian representatives of 

 the group have been studied. 



The systematic study of the Diplopoda is a difficult and backward 

 province of zoology ; the literature is fragmentary and scattered, and 

 the animals, though quite harmless, are generally avoided by all except 

 the most intelligent and zealous collectors. The Diplopods of the 

 Pacific Coast region have had little attention subsequent to the papers 

 of Wood and Harger, published over thirty years ago, and it has ac- 

 cordingly seemed permissible to insert here some of the preliminary 

 results of a long-deferred examination of the considerable amount of 

 material accumulated at Washington during the last decade. 



As a means of encouraging the collection of Diplopoda in the North- 

 west, analytical keys to the higher groups and general notes likely to 

 facilitate diagnosis have been included in the present report. The 

 characters used for these purposes have been chosen with a view to 

 convenience, and are not in all cases of general application to the 

 Diplopoda of other regions. 



ANALYTICAL KEY TO THE SUBCLASSES OF DIPLOPODA. 



Body without external armor ; the soft skin beset with toothed and 

 barbed bristles Subclass PSELAPHOGNATHA. 



Body with a hardened exoskeleton ; bristles few, inconspicuous, or 

 wanting Subclass CHILOGNATHA. 



Subclass PSELAPHOGNATHA. 



The subclass Pselaphognatha contains only the small caterpillar-like 

 Diplopod Polyxenus and a few related genera. The group is of very 

 general distribution and is probably very old geologically, one of the 

 Carboniferous forms {Palceocampa) being of large size. A species 

 of Polyxenus from the State of Washington has been described by 



