1.	Apical margin of clypeus strongly thickened (Figs 11, 12)	Osmia nigriventris (Zetterstedt)
–	Apical margin of clypeus more or less flat (Figs 4, 6)	2
2.	Ventral margin of mandible with distinct tooth (Fig. 52) (propodeal triangle strongly granulose, Fig. 56; mandible with apical margin about a third wider than median width, Fig. 4)	Osmia inermis (Zetterstedt)
–	Ventral margin of mandible sometimes slightly swollen medially, but lacking distinct tooth	3
3.	Propodeal triangle strongly granulose (Figs 16, 56)	4
–	Propodeal triangle with more or less shining ventral area (Figs 36, 57) (mandible with apical width 1.5 times greater than median width) [xanthomelana species group]	6
4.	T2–T3 with apical impunctate bands nearly one-third of postgradular width	Osmia tersula Cockerell (western form)
–	T2–T3 with apical impunctate bands lacking or about one-fifth of postgradular width [inermis species group]	5
5.	Mandible with third tooth broad, neither strongly protruding nor strongly separated from fourth tooth (Fig. 6), with condylar and outer ridges strongly converging apically (Fig. 5); mesepisternum with white to pale yellow hairs; T1 dorsal surface with distinct punctures (Fig. 58)	Osmia laticeps Thomson
–	Mandible with third tooth symmetrically triangular, broadly separated from both fourth and second teeth by acute-angled and broad indentation (Fig. 2), with condylar and outer ridges nearly parallel to weakly converging apically (Fig. 1); mesepisternum with hairs predominantly blackish, only on its most anterior and dorsal part with narrow zone bright yellowish-brown; T1 dorsal surface granulose/papillate, with punctures less strongly impressed (Fig. 17)	Osmia aquilonaria Rightmyer, Griswold, & Arduser, sp. n.
6.	Mandible with third tooth in same plane as second and fourth teeth, lacking distinct carina separating it from second and fourth teeth (Fig. 10); outer hind tibial spur weakly curved apically; clypeus with apical truncate process with distinct lateral angle, margin lateral of truncation distinctly stairstepped (Fig. 35); hair fringe of galea in dorsal view shorter than width of malar space at mandibular condyle	Osmia nearctica Rightmyer, Griswold, & Arduser, sp. n.
–	Mandible with third tooth distinctly recessed between second and fourth teeth, with carina separating it from second and fourth teeth (Fig. 8); outer hind tibial spur strongly curved apically; clypeus with apical truncation not distinctly angled laterally, margin lateral of truncation sinuate but not distinctly stairstepped (Fig. 55); hair fringe of galea in dorsal view as long as width of malar space at mandibular condyle	Osmia maritima Friese