302 HTMENOPTERA OF AMERICA. [PART I. 



The difficulty in distinguishing tlic Stenodynerus from the 

 PacJiodynerus and the true Odynerus obliges us to make a review 

 of the characters which are peculiar to each. 



The clypeus is not characteristic in any. Generally pyriform, 

 truncate, and biangulate at tip; in ? sometimes rounded; in % 

 polygonal-truncate and bidentate. 



The thorax is cylindrical when the abdomen is elongate- 

 cylindrical, vaulted above, anteriorly not contracted, biangulate 

 (0. eonformis, toltecus, perennis, etc.). When the abdomen is 

 more ovate or spindle-shaped, the th®rax is not so elongate, 

 more triangular, contracted behind. It is nearly always more 

 or less coarsely punctured, the metathorax being less strongly 

 punctured. 



The post-scutel is very peculiar. It has two different forms: 



1st. Not truncate; that is, angulate behind, not forming a 

 superior transverse punctate face and a posterior vertical polished 

 face, but on the contrary entire ; rather horizontal or oblique, 

 falling into the hinder face of metathorax, more elevated anteriorly 

 than posteriorly, as if it had been a little squeezed from behind 

 forwards, so as to accumulate it somewhat along the anterior 

 margin ; when crested, the crest is placed on this elevated part 

 near the anterior margin, and not, as in the Divis. Odynerus, on 

 the posterior edge formed by the truncation. When not trun- 

 cate, it is punctured on its whole surface. 



2d. Truncate posteriorly and crested on the ridge of the trun- 

 cation, nearly as in Divis. Odynerus. Cut in this case the trun- 

 cation is more oblique, as if impressed from behind, and the crest 

 rather pushed forward against the anterior margin. The trunca- 

 tion also is not as straight transversely, but is rather arcuate. 

 In case the scutel is truncate, the Stenodynerus arc yet quite 

 distinct from Divis. Odynerus, in having the metathorax quite 

 convex, not widely excavated, so that the posterior face of the 

 post-scutel does not, as in Odynerus, make a part of the posterior 

 concavity of the metathorax, and besides, the abdomen is quite 

 slender, spindle-shaped, the first segment being funnel-shaped, 

 rather pediculate, not wide and sessile as in the Divis. Odynerus. 



The metathorax is quite variable. 1st. Sometimes produced 

 horizontally, further than the post-scutel, including it behind 

 (0. luctuosus), as in some Ancistrocerus {A. i-seetus). 2d. Gene- 

 rally not produced superiorly, but somewhat produced under the 



