12 Myron Harmon Sivenk 



which is subject to much intergradation, yet in some species re- 

 ferred to Noniada sens. str. {e. g., propinqua) the tips of the 

 mandibles are sometimes truncated and there is a deep median 

 groove or even a slight terminal notching at the end of this 

 groove. Such specimens, though they suggest the derivation of 

 Gnathias, do not approach at all closely to the species actually 

 referred to that group in the mandibular character, and are easily 

 referred to typical Nomada. In all other characters except the 

 bidentate mandibles Gnathias may be matched by species of 

 typical Nomada. 



Robertson's alleged genus Phor, to which only the type species 

 A^. intcgerrima D. T., from Illinois, and A^ subgracilis Ckll., from 

 California, have been referred, and to which A'', vexator Ckll., 

 from Colorado obviously belongs, is fairly intermediate between 

 Nomada sens. str. and Holonomada. The entire apex of the 

 male separates it from any species of Nomada sens. str. and 

 indicates affinity with Holonomada, or at least a parallel differ- 

 entiation, while the coloration, antennal structure, etc., are all 

 those of typical Nomada in both sexes, much like that found in 

 the accepta group. The black curved bristles of the apex of the 

 hind tibiae of the female occur in various species of Nomada 

 (e. g., bisetosa) while numerous species of Nomada have the 

 basal nervure interstitial with the transverso-medial nervure. 

 The group can probably best, be disposed of by recognizing it as 

 a subgenus, at least for the present. 



Robertson in characterizing his genus Cephen expressed some 

 doubt as to its distinctness from Micronomada, but because of 

 the more narrowed second submarginal cell, more pointed 

 marginal cell and basal nervure interstitial with or slightly basad 

 of the transverso-medial in the type species of Micronomada as 

 compared with the type species of Cephen, he allowed the latter 

 to stand. A study of a series of modesta shows considerable 

 variation in these characters, and when allied species are con- 

 sidered the distinctions become worthless as generic or subgeneric 

 characters, being, indeed, scarcely of specific value. Cockerell 

 has repeatedly stated his inability to distinguish Cephen from 



12 



