Approximately half way down the crus, the three heads give rise to the tendon of insertion, the _tendo achillis_, which passes over and is tightly bound to the posterior surface of the tibial cartilage. The insertion is tendinous on the posterior surface of the hypotarsus and along the posterolateral ridge of the tarsometatarsus. This tendon seems to be continuous with a fascia which forms a sheath around the posterior surface of the tarsometatarsus holding the other tendons of this region firmly in the posterior sulcus. Action.--Extends tarsometatarsus. Comparison.--Study of the _pars externa_ and _pars media_ reveals no significant differences among the species dissected. The _pars interna_, however, is subject to some variation which is described below. _Pars interna_ bipartite _Vireo_ _Chlorura_ _Seiurus_ _Pipilo_ _Icterus_ _Calamospiza_ _Molothrus_ _Chondestes_ _Piranga_ _Junco_ _Richmondena_ _Spizella_ _Guiraca_ _Zonotrichia_ _Passerina_ _Passerella_ _Spiza_ _Calcarius_ The two parts of the _m. gastrocnemius_ are most distinct in _Vireo_. _Icterus_, _Molothrus_, _Richmondena_, _Guiraca_, and _Passerina_ lack the fibrous band that passes around the front of the knee. In _Spiza_ this band of fibers is smaller than in the other species. _Pars interna_ undivided _Passer_ _Pinicola_ _Estrilda_ _Leucosticte_ _Poephila_ _Spinus_ _Hesperiphona_ _Loxia_ _Carpodacus_ In _Leucosticte_, although the _pars interna_ is undivided, there is a band of fibers which extends around the front of the knee (see discussion, p. 183). _+Musculus plantaris+_ (Fig. 5).--Small and slender, this muscle lies on the posteromedial surface of the crus, beneath the _pars interna_ of the _m. gastrocnemius_ and originates by fleshy fibers from the posteromedial surface of the proximal end of the tibia immediately distal to the internal articular surface. The belly extends approximately one-sixth of the way down the crus and gives rise to a long, slender tendon that inserts on the proximomedial edge of the tibial cartilage. Action.--Extends tarsometatarsus. Comparison.--No significant differences noted among the species studied.