1.	T5 and T6 with apicolateral angles strongly reflexed laterally (Fig. 53)	Osmia nigriventris (Zetterstedt)
–	T5 and T6 with apicolateral angles not or only weakly reflexed laterally	2
2.	Outer margin of gonoforceps preapically with semicircular widening, at this point gonoforceps appearing nearly twice as broad as its narrowest width (Figs 45, 49, 50, 64) [xanthomelana species group]	3
–	Outer margin of gonoforceps preapically not or only weakly widened, at this point gonoforceps at most little broader than at its narrowest width (Figs 26, 30, 31) [inermis species group]	4
3.	Flagellar segments on ventral surface with hairs microscopic; S2 with midapical margin not emarginate	Osmia nearctica Rightmyer, Griswold, & Arduser, sp. n.
–	Flagellar segments on ventral surface with sparse hairs, their length about half the flagellar segment width; S2 with midapical edge weakly emarginate	Osmia maritima Friese (based on observed palearctic material only)
4.	S4 with hooked bristles both along apical margin and on premarginal area, along apical margin the bristles oriented horizontally and on premarginal area directed increasingly vertically (Figs 24, 25)	Osmia aquilonaria Rightmyer, Griswold, & Arduser, sp. n.
–	S4 with midapical hairs unmodified, without hooked bristles (Figs 60, 61)	5
5.	S4 with apical margin truncate, medially with strong emargination and distinct, rounded lobes lateral to emargination (Fig. 60); declining basal portion of T1 densely shagreened, only with a silky luster	Osmia inermis (Zetterstedt)
–	S4 with apical margin more or less evenly convex, lacking strong emargination at midpoint and distinct sublateral lobes (Fig. 61); declining basal portion of T1 shining, at most superficially shagreened in small areas	Osmia laticeps Thomson