Sept., 1905.] Roberts: Species of Dytiscus and Cvbister. 105 



Of these we have in the east dauricus only. 



Group III. — Labrum truncate in middle ; apices of coxal proc- 

 esses acutely spinose ; margins of elytra dilated. America is not rep- 

 resented in this group, the one species being from northern Europe. 



Group IV. — Labrum nearly truncate in middle; apices of coxal 

 processes obtuse ; margins of elytra nearly simple. 



Represented by one species, harrisii, which is found here in the East. 



Of Cybister we have one species here, fimbriolatus, easily separated 

 from Dytiscus by the generic characters and by its general form, 

 which, instead of being regularly ovate, is more or less wedge-shaped 

 on account of its being quite rapidly narrowed from base of thorax to 

 head, and gradually broadened from base of thorax to one third from 

 apex of elytra. 



Having eliminated Cybister fimbriolatus ; the only eastern Cybister, 

 this leaves us with six species of Dytiscus to differentiate, namely : 

 fasciventris, hybridus, verticalis, marginalis, dauricus and harrisii. 

 At a casual glance the three most likely to be confused are verticalis, 

 dauricus and harrisii, as all are of large size and similar form. 



We can easily distinguish verticalis from the other two by the 

 absence of a yellow apical and basal thoracic margin, and we can at 

 once separate dauricus from harrisii by the form of the coxal proc- 

 esses. In harrisii they are rounded and blunt, in dauricus pro- 

 duced into a sharp spine. There is also the group character in the 

 shape of the clypeus — dauricus distinctly emarginate ; harrisii 

 nearly truncate or scarcely at all emarginate. 



The only other species likely to be confused with these is mar- 

 ginalis, but it is smaller than the other three and the group characters 

 will separate it, it combining half of each, but agreeing with neither. 

 The labrum being distinctly emarginate distinguishes it from harrisii 

 and the coxal processes being rounded, not acutely spinose, separates 

 it from dauricus, thus throwing it into the first group with verticalis, 

 hybridus and fasciventris. 



From the other three in group I, margin'alis can be distinguished 

 by the yellow thoracic markings. Fasciventris has scarcely a trace, 

 or none at all, of a basal or apical yellow margin, and verticalis has 

 none at all ; hybridus has a fine linear yellow line at base and a fairly 

 distinct margin in same color at apex, while in marginalis the basal 

 yellow margin is distinct all the way across and specially broad in its 

 middle portion. 



