_+Musculus flexor perforatus digiti IV+_ (Fig. 3).--Extending along the posterior edge of the crus, this slender muscle lies beneath the _m. gastrocnemius_. The belly is fused with those of the _m. flexor hallucis longus_ and _m. flexor perforatus digiti III_. Its origin is fleshy from the intercondyloid region of the distal end of the femur and has a few fibers arising from the tendon of origin of the _m. flexor perforatus digiti III_. Near the distal end of the crus the muscle gives rise to the strong tendon of insertion which perforates the tibial cartilage near its lateral edge and in this region is ensheathed by the tendon of the _m. flexor perforatus digiti III_. The two tendons pass together through the posterolateral canal of the hypotarsus (Fig. 6). The tendon continues distally along the tarsometatarsus and the posterior surface of digit IV. The tendon bifurcates at approximately the middle of the first phalanx. A short lateral branch inserts on the posterolateral edge of the proximal end of the second phalanx. The long medial branch is perforated by a branch of the _m. flexor digitorum longus_; the distal end is flattened, has thickened edges, and inserts over the posterior surfaces of the distal end of the second phalanx, and over the proximal end of the third phalanx. Action.--Flexes digit IV. Comparison.--No significant differences noted among the species studied. _+Musculus flexor perforans et perforatus digiti II+_ (Figs. 2, 9).--Small and spindle-shaped, this muscle lies on the posterolateral side of the crus immediately beneath the _pars externa_ of the _m. gastrocnemius_. The origin is fleshy and arises in company with the _m. flexor perforans et perforatus digiti III_ from a point on the posterolateral surface of the distal end of the femur between the point of origin of the _pars externa_ of the _m. gastrocnemius_ and the fibular condyle. The belly extends approximately one-fourth of the way down the crus and gives rise to the tendon of insertion which passes distally and superficially through the posterior edge of the tibial cartilage. The tendon traverses the posteromedial canal of the hypotarsus (Fig. 6) and continues along the posterior surface of the tarsometatarsus. Between the first metatarsal and the base of the second digit the tendon is enclosed by the medial surface of a sesamoid bone. This tendon then perforates that of the _m. flexor perforatus digiti II_ at the level of the first phalanx and in turn is perforated by the tendon of the _m. flexor digitorum longus_ at the proximal end of the second phalanx. The insertion is on the posterior surface of the second phalanx. Action.--Flexes digit II. Comparison.--In _Passer_, _Estrilda_, _Poephila_, _Hesperiphona_, _Carpodacus_, _Pinicola_, _Leucosticte_, _Spinus_, and _Loxia_ the proximal portion of this muscle is more intimately connected with the posterior edge of the _m. flexor perforans et perforatus digiti III_ than it is in the other species examined.