The dark or grayish bodied horseflies, superficially resembling Tabanus rhombicus O. S. or its variety osburni Hine, are among the abundant and trouble some species encountered in western and boreal North America. Four new species and the previously undescribed males of 6 others, together with a rather widely scattered literature have suggested the value of bringing together some notes on this rather heterogeneous group. Most of the species have 3 rows of pale markings on the abdomen, but a few are black with little or no indication of pattern; all here included have been placed in the subgenus Therioplectes, with hairy eyes in both sexes and an ocelligerous tubercle in the females. The numbers of specimens actually seen by me from various western states are included.