armatures in the Hymenoptfrous genus CoUetes. 57 



Their middle part has a more or less bulging external out- 

 line — less noticeable, however, in this particular species 

 than in some near it, e. g. montamts. The sagittte have a 

 very noticeable dentiform dilatation between the basal 

 dilatation and the " wing." The pilosity at the apex of 

 each stipes is decidedly rather long and copious. 



The seventh ventral segment in viarginatus has a distinct 

 facies of its own. Its outlines are rather angular than 

 rounded — note especially the lateral projections towards 

 its base. It is mostly nearly colourless, but with yellowish 

 costal thickenings. Though it has certain points of like- 

 ness to the same segment in two or three of the other 

 species, it could hardly, I think, be confused with any of 

 them. 



It is, perhaps, scarcely necessary to repeat that Rados- 

 zkowski's figure of " marginata, Smith," has nothing to 

 do with this species, but represents one of the halteatus 

 group. 



On the other hand, halteatus, Rad. {ncc. Perez), certainly 

 does represent our British species, his type having been 

 received (as he tells us) as marginattis from Mr. E. 

 Saunders, and his figure, though hardly good, being at 

 least recognizable. 



25. Mongolicus, Perez. PI. VIIL 25, 25«. 



I have received this species from the author, and also 

 (unnamed) from Herr Kohl. 



Its armature closely resembles that of mctrginatus, but 

 the seventh ventral plate distinguishes it at a glance. This, 

 alone among the species with "divided stipites," has the 

 sort of outline (long petioles and bilobed apices) which 

 characterizes the halteattts group, and yet it could hardly 

 be confused with any of them. 



I cannot point to any case which illustrates better than 

 the present the value of the characters of the seventh ventral 

 plate for determination of obscure Colletes species. 

 Mongolicus and marginatus both occur in Mongolia, and 

 their armatures are practically not distinguishable. Yet 

 by the seventh ventral plate, as well as by sufficient though 

 minute external characters, they are unmistakably shown 

 to be distinct species. And in a third Mongolian species, 

 to be dealt with below, we have again the armature of 

 mongolicus and marginatus, but a seventh ventral plate 

 which separates it at once from either of them. 



