380 PROCEEDINGS OF THE NATIONAL MUSEUM vol. lu 



3b. Terminal process a narrow, shallow trough with extreme apex convex 



and recurved (fig. 91) Macroporus 



26. Terminal process short, transverse length not more than twice width. 

 4a, Metapleural evaporatorium limited, simply outlining peritreme (fig. 90). 



Microporus 

 46. Metapleural evaporatorium extensive, occupying most of segment 

 (figs. 95-97). 

 5a. Terminal process large, elongate-oval, with one to three longitudinal 



rugae discally (fig. 89) Cydnus 



56. Terminal process not elongate-oval, without rugae (figs. 95-97, 101). 



Ectinopus; Melanaethus; Onalips 



16. Anterior part of osteolar peritreme not differentiated terminally (figs. 



102-112), posterior part sometimes with spineHke or tonguelike process; 



osteole opening posteriorly on peritreme, not visible ventrally. 



6a. Pronotum with a sharply defined, deeply impressed transverse line 



paralleling anterior margin from side to side (figs. 14, 73) . . Pangaeus 



66. Pronotum without such a line. 



7o. Posterior tibia strongly compressed, spines confined to dorsal and 

 ventral margins, ventral spines longer, thinner and more tapering 

 than those of dorsal margin (figs. 141, 142). 

 8a. Labial II with a semicircular foliaceous lobe (fig. 36) . . Prolobodes 



86. Labial II without such a lobe (fig. 34) Cyrtomenus 



76. Posterior tibia not compressed, spines rather uniformly developed on 

 all margins (figs. 140, 148-150) Dallasiellus; Tominotus 



In the above arrangement, two points are worthy of mention. 

 First, the two genera Hsted under 76 represent those which were stated 

 above to be very difficult to separate fully and satisfactorily. As 

 previously mentioned, this is a "residual area" of negative characters 

 that includes a number of species groups. But whether these groups 

 are worthy of generic, subgeneric, or even lower standing is not yet 

 evident. For convenience they are held thus. Supporting evidence 

 will be found in the generic discussions of them. 



The second noteworthy point is the absence of certain familiar 

 generic names like Aethus, Geocnethus, and Gcotomus. These three 

 genera have Old World genotypes and none of our forms is con- 

 generic with them. In general, our species formerly assigned to 

 Aethus belong to Tominotus; those listed as Geocnethus go to Dalla- 

 siellus; and the name Melanaethus replaces Geotomus in the Western 

 Hemisphere. These name changes are discussed under the appro- 

 priate generic discussions. With these and certain other generic 

 redefinitions resulting from a companion study on the Old World 

 forms, each genus now assumes a zoogeographic significance that it 

 formerly lacked. 



Key to genera of Cydninae known in the Western Hemisphere 



1. Anterior part of osteolar peritreme modified apically into a distinctly differ- 

 entiated loop, lobe, or band which is wider than basal part of peritreme 

 and more or less polished (figs. 89-101) 2 



