The _pars posticus_ originates by a broad, flat tendon on a line across the posterior half of the membrane that covers the ischiopubic fenestra. The insertion is at the point of origin of the _pars media_ of the _m. gastrocnemius_ on the posteromedial surface of the proximal end of the internal condyle of the femur. There is a broad tendinous connection with the proximal end of the _pars media_ of the _m. gastrocnemius_. The anterior edge of the _pars posticus_ is overlapped medially by the posterior edge of the _pars anticus_. Action.--Flexes thigh; may flex crus also and may extend tarsometatarsus. Comparison.--In _Vireo olivaceous_, the origin of this muscle does not extend the length of the ischiopubic fenestra. The origin, furthermore, is along the dorsal edge of the ischiopubic fenestra and not from the membrane covering the fenestra. Finally, in this species, the origin of the _pars posticus_ is fleshy. _+Musculus tibialis anticus+_ (Figs. 2, 5).--Lying along the anterior edge of the crus, a part of this muscle is covered by the _m. peroneus longus_. The origin is by two distinct heads, each of which is pinnate. The anterior head arises directly from the edges of the outer and inner cnemial crests. The posterior head arises by a short, strong tendon from a small pit on the anterodistal edge of the external condyle of the femur. This tendon and the proximal end of the muscle pass between the head of the fibula and the outer cnemial crest. The two heads of the muscle fuse at a place slightly more than one-half of the distance down the crus. At the distal end of the crus this muscle gives rise to a strong tendon which passes under a fibrous loop immediately proximal to the external condyle in company with the _m. extensor digitorum longus_ and which passes between the condyles of the tibia and inserts on a tubercle on the anteromedial edge of the proximal end of the tarsometatarsus. Action.--Flexes tarsometatarsus. Comparison.--No significant differences noted among the species studied.