In preparing specimens for study, the legs and pelvic girdle were removed and washed in running water for several hours to remove much of the formalin. They were then transferred to a mixture of 50 per cent alcohol and a small amount of glycerine. All specimens were dissected with the aid of a low power binocular microscope. Where possible, several specimens of each species were examined for individual differences. Such differences were found to be slight, involving mainly size and shape of the muscles. The size is dependent partly on the age of the bird, muscles from older birds being larger and better developed. The shape of a muscle (whether long and slender or short and thick) is due in part to the position in which the leg was preserved; that is to say, a muscle may be extended in one bird and contracted in another. For these reasons, descriptions and comparisons are based mainly on the origin and insertion of a muscle and on its position in relation to adjoining muscles. Birds dissected in this study are listed below (in the order of the A. O. U. Check-List): SPECIES _Vireo olivaceus_ (Linnaeus) _Leucosticte tephrocotis_ _Seiurus motacilla_ (Vieillot) (Swainson) _Passer domesticus_ (Linnaeus) _Spinus tristis_ (Linnaeus) _Estrilda amandava_ (Linnaeus) _Loxia curvirostra_ Linnaeus _Poephila guttata_ (Reichenbach) _Chlorura chlorura_ (Audubon) _Icterus galbula_ (Linnaeus) _Pipilo erythrophthalmus_ _Molothrus ater_ (Boddaert) (Linnaeus) _Piranga rubra_ (Linnaeus) _Calamospiza melanocorys_ _Richmondena cardinalis_ (Linnaeus) Stejneger _Guiraca caerulea_ (Linnaeus) _Chondestes grammacus_ (Say) _Passerina cyanea_ (Linnaeus) _Junco hyemalis_ (Linnaeus) _Spiza americana_ (Gmelin) _Spizella arborea_ (Wilson) _Hesperiphona vespertina_ (Cooper) _Zonotrichia querula_ (Nuttall) _Carpodacus purpureus_ (Gmelin) _Passerella iliaca_ (Merrem) _Pinicola enucleator_ (Linnaeus) _Calcarius lapponicus_ (Linnaeus) Description of Muscles The descriptions which follow are those of the muscles in the leg of the Red-eyed Towhee, _Pipilo erythrophthalmus_. Differences between species, where present, are noted for each muscle. The term thigh is used to refer to the proximal segment of the leg; the term crus is used for that segment of the leg immediately distal to the thigh. _+Musculus iliotrochantericus posticus+_ (Fig. 2).--The origin of this muscle is fleshy from the entire concave lateral surface of the ilium anterior to the acetabulum. The fibers converge posteriorly, and the muscle inserts by a short, broad tendon on the lateral surface of the femur immediately distal to the trochanter. It is the largest muscle which passes from the ilium to the femur. Action.--Moves femur forward and rotates it anteriorly. Comparison.--No significant differences noted among the species studied.