The orthopyrcxenes of the Bushveld Complex, Transvaal; Stillwater Complex, Montana; Bay of Islands Complex, Newfoundland; Great Dyke of Southern Rhodesia; peridotites of Cuba and most of the other mafic plutonic bodies thus far examined by the writers, have a peculiar striated or laminated appearance in a thin section and exhibit certain abnormal extinction angles. Much confusion concerning these pyroxenes exists in the literature. Niggli, Johannsen, Scholtz, and Chudoba, for example, state that the optic plane is parallel to 100, whereas Larsen, Winchell, Dana, and others place the optic plane of orthopyroxenes parallel to 010. † The laminated appearance and small extinction angles have been considered to be the result of twinning on a very fine or sub-microscopic scale; and the mineral, though outwardly appearing to be orthorhombic, is interpreted as monoclinic. Chudoba suggests that the lamination is due to polysynthetic twinning with translation parallel to 100 (1).