BIRDS OF NORTH AND MIDDLE AMERICA. 329 



diagnosis than that given above is scarcely practicable. Indeed, our 

 knowledge of their internal structure is limited to so few of the forms 

 that the extent to which our present ideas of the classification of the 

 group may require modification after the anatomy of additional 

 genera has been studied is a matter of great imcertainty; in fact it is 

 not at all improbable that many changes in the limitations and com- 

 position of the family groups may be necessary. The anacromy- 

 odous syrmx and homoeomerous tliigh artery of many genera of 

 Tyrannidae as well as the catacromyodous syrinx and heteromerous 

 thigh artery of many genera of Piprid^ and Cotingidse are, for exam- 

 ple, merely assumed, and there may be many exceptions to these sup- 

 posedly diagnostic characters of the groups in question. Certainly 

 some genera referred to the Cotingidse can be separated, so far as 

 external features admit, from the TjTannidse only by their non- 

 exaspidean tarsi; but at the same time the character of the tarsal 

 envelope is hardly more different than it is from that of certain other 

 menibers of the Cotingidap. In the absence, therefore, of Ivnowledge 

 as to the insertion of the vocal muscles and other anatomical char- 

 acters of certain genera w^e must necessarily for the present be guided 

 largely by external characters, and therefore the classification indi- 

 cated in the following key to the famiHes must be regarded as purely 

 tentative. 



The Mesomyodi are cliiefly American and mostly Neotropical, only 

 three families (Xenicidge, Pliilepittidse, and Pittidae) occurring m the 

 Eastern Plemisphere. Notwithstanding the difference in structure of 

 their vocal organs many species (notably among the FormicariidaB) 

 are songsters of considerable merit. 



have the tarsal envelope for its greater part typically exaspidean, but on the upper 

 portion the acrotarsium extends only part way across the outer side, where there may 

 1)0 either distinct posterior series of scutella or small roundish or oval scutella extend- 

 ing for a short distance below the heel joint, or this space may be entirely nonscutellate. 

 The two edges of the acrotarsium may be in actual contact for a greater or less portion 

 of the length of the tarsus, or there may be a space of greater or less width between; 

 this space being usually nonscutellate, but sometimes partly occupied by small roimd- 

 ish scutella, the tarsus then being semi-pycnaspidean. Sometimes the scutella are 

 fused, the smooth (ocreate or ' 'booted '' ) tarsal covering then superficially resembling 

 that of certain Oscines; l)ut careful comparison will show a radical difference in the 

 forv) of the post(>rior side of the tarsus, which in all Oscines except tlie Alaudidic (in 

 which the tarsus is holaspidean) has the posterior margin contracted into a sharp or 

 narrow lidge or edge. 



