122 Bulletin American Museum of Natural History. [Vol. XXXVI, 



ern forms is surprisingly large. It is, however, to be especially noted that 

 whereas many of the Amazonian forms have crossed the Cauca-Magdalena 

 district and entered Panama and even Central America, no distinctly 

 Pacific coast species appears to have gone east over the route at the northern 

 end of the Eastern Andes by which Amazonian species have traveled west- 

 ward. This fact might indicate that in spite of the proximity of the district 

 whence it has been derived, the west coast element is of more recent origin, 

 but a comparison of the changes which have occurred in both groups since 

 their establishment in the humid Cauca-Magdalena Fauna does not con- 

 firm this theory, species of western origin showing as much racial variation 

 as those from the east. 



The abrupt cessation of forest growth on the floor of the Magdalena 

 Valley at La Dorada marks the southern limit of the range in that valley of 

 the forest-inhabiting species which characterize the humid Cauca-Magda- 

 lena Fauna. Tropical Zone forest extends at least as far south as the lati- 

 tude of Honda, on the slopes of the mountains, carrying with it such forest- 

 loving species as Myrmelastes immaculatus and Formicarius analis saturatus, 

 but beyond this the avifauna of the Tropical Zone of the upper Magdalena 

 Valley is composed of species which frequent plains, thickets and low scrubby 

 woods. Examples are Colinus cristata leucotis, Ortalis columhiana 

 Columbiana, Psittacula conspicillata conspicillata, Thamnophilus radiatus 

 albicans, Myrmeciza I. boucardi, Arremenops conirostris conirostris and A. c. 

 incxpectata, Thraupis c. cana, Thraupis palmarum melanoptera, etc. With 

 but few exceptions all the Tropical Zone species inhabiting the upper 

 Magdalena Valley have evidently entered it from the north passing the 

 forested area lying between Banco and La Dorada. Its life, therefore, 

 resembles that of the arid lower Magdalena or Caribbean Fauna, rather 

 than that of the Tropical Zone at the eastern base of the Eastern Andes in 

 the same latitude. 



Not only have these mountains proved a barrier to extension of range 

 directly over them, but of equal, or in view of the low altitude of the Anda- 

 lucia Pass (7000 ft.) of possibly greater importance, is the lack of forests 

 in the upper Magdalena which would afford a favorable home for the spe- 

 cies inhabiting the densely wooded region at the eastern base of the moun- 

 tains. 



In several instances, however, notably with species not so strictly con- 

 fined by zonal boundaries as is customary, it is evident that forms of the 

 extreme upper Magdalena Valley have entered it from the east over the 

 mountains. The known examples are Piaya cayana mesura (upper Mag- 

 dalena specimens agreeing with those of the eastern slope of the Andes in- 

 stead of with those of the vicinity of Honda), Conopophaga castaneiceps 



