The division of the _pars interna_ of the _m. gastrocnemius_ into anterior and posterior parts has not been reported by previous authors yet the division is quite distinct in those birds in which it occurs. Hudson (1937:36) points out that in some non-passerine birds the _pars interna_ is double, but that in these species the _m. semimembranosus_ inserts between the two parts. This is not the condition in those species studied by me. Only the ploceids and the cardueline finches in the present investigation fail to show such a division. The undivided muscle in these birds resembles, in its origin and position, the posterior portion of the muscle found in those species showing the bipartite condition. The greater mass of the bipartite muscle probably makes possible a stronger extension of the tarsometatarsus. Thus, the divided or undivided conditions of the _m. obturator externus_ and the _pars interna_ of the _m. gastrocnemius_ seem to be correlated with the degrees of strength of certain movements of the leg. It is conceivable that these differences in structure are correlated with the manner in which food is obtained, the birds having the bipartite muscles being those which spend the most time on the ground searching and scratching for seeds and other sorts of food. Yet, in _Leucosticte_, a cardueline, and in _Calcarius_, an emberizine, whose foraging habits are rather similar, the structure is unlike. _Leucosticte_ does resemble the emberizines and also _Piranga_ and _Spzia_ in the extension of a band of muscle fibers from the _pars interna_ of the _m. gastrocnemius_ around the front of the knee. A band of muscle fibers of this sort strengthens the knee joint and gives still more strength to the _pars interna_. This condition has been reported in a number of birds by Hudson (1937) and is, in all probability, an adaptation for greater strength of certain leg movements. The development of this band in _Leucosticte_ seems to parallel that in the other birds studied and does not indicate relationship, since in _Leucosticte_ this band arises from the undivided muscle which (as stated above) resembles only the posterior portion of the bipartite muscle described for the other birds. In the latter, the muscular band arises from the anterior part of the muscle.