398 Dr. A. S. Packard, Jr's Revision of the 



the form of this area in the Philanthinse and Crabroninas; also 

 in the stoutly jointed and very clavate geniculate antennae, characters 

 which prevail in the Nyssonidae, Larridae, Sphegidae and allied fosso- 

 rial groups, together with the peculiar neuration and form of the front 

 of the head, reminding us throughout of Oxijbelus and Nj/sson. 



The neuration of Psen differs very considerably in the different 

 species. Thus Psen ater Fabr. presents notable differences in the 

 neuration from both of the two species described below, since the two 

 median cells are greatly shortened, thus imparting a very different 

 facies to the entire character of the neuration. This shows how cau- 

 tious we must be in the use of these alary characters in our generic diag- 

 noses. The eyes in Psen are much larger than in Pemphredon, Ce- 

 monus or Passaleccus. 



Compared with the other sex, the male of this genus has a much more 

 transverse head, which rapidly contracts in width posteriorly; the 

 vertex is much more elevated, clypeus shorter, antennas much longer, 

 subfiliform, and the legs are slenderer, while the abdomen has a longer 

 pedicel. 



Synopsis of the Species. 



A. Front silvery, antennae black. 

 % . Black, tarsi whitish. 



9. Black, pedicel shorter than the width of the antennae P. leucopus, Say. 



% . Pedicel more than £ as long as the abdomen, feet fus- 

 cous. 



9. Pedicel as long as the abdomen is wide, species larger 



than the preceding P. niger, Pack. 



9 . Abdomen very long and flattened, sutures very distinct..?. elongatus,Paek. 



B. Front golden. 



9. Antennae fuscous beneath, body stout P. chalcifrons, Pack. 



9 . Of large size, flanks golden; % basal abdominal rings 



red on the hinder edge P. fuscipes, Pack. 



Psen leucopus Say. 



Psen leucopus Say, Descr. New Species N. Amer. Hym. Bost. Journ. Nat. 

 Hist. 1, May, 1837, p. 370. 



% . Black, head rather sparsely punctured above, and polished; 

 below, thickly pubescent, silvery ; scape rather hirsute, eyes smaller 

 and front much narrower, with more space between the eyes and supra 

 clypeal region, which area is natter than in 9 . Mandibles and anten- 

 nae black. Thorax and propodeum as described in the 9 , but the 

 mesial furrow of the latter is much deeper, and posteriorly very 

 broad and sunken, forming a deep channel, on each side of which rises 

 the somewhat tumid sides; sides of thorax sericeous, which are 





