146 BuUetin American Museum of Natural History. [Vol. XXXVI, 



Peruvian region, northward through Colombia. To it should doubtless be 

 added the subtropical western slope of the Central Andes which, while far 

 from possessing all the forms that characterize the West Andean Subtropical 

 Fauna, has a closer affinity with that fauna than it has with the East Andean 

 Subtropical Fauna. This is indicated by the presence of such distinctive 

 West Andean species as Geotrygon hourcieri, Formicarius rufi/pectus carrikeri, 

 Chlorochrysa nitidissima and Ostinops sahnoni. Furthermore, with species 

 which have representative races in the West Andean Fauna and the East 

 Andean Fauna, the form of the western slope of the Central Andes usually 

 agrees with that of the Western Andes, that of the eastern slope with that 

 of the Eastern Andes. Taking the Central Andes, as a whole, however, 

 the East Andean Subtropical element is much stronger than that received 

 from the Western Andes, a fact obviously attributable to existing topography 

 and to the humid connection at the head of the Magdalena Valley. 



On the Pacific slopes of the Andes this faunal belt stretches continuously 

 from its southern end to northern Colombia. Whether it exists on the 

 summit of the Baudo range unfortunately is not known. It reappears in 

 dilute form on the crests of the higher mountains of eastern Panama and 

 Costa Rica, and its influence extends even to southern Mexico. Its appar- 

 ent absence between the higher portions of eastern and western Panama has 

 already been mentioned and will be referred to in detail later. 



While apparently always present on the western slope of the Western 

 Andes, it is developed on the eastern slope of this range only above an eleva- 

 tion of 6500 feet; the altitude of condensation, as explained in writing of the 

 Tropical Zone, being higher on the eastern than on the western slope of this 

 range. 



Its forests stretch, apparently without a break, along the western slopes 

 of the Central Andes above the Cauca Valley, are wanting in southern 

 Antioquia, but reappear in the more northern part of that department. 

 Here the Western and Central Andes are separated only by the Cauca River 

 from opposite banks of which they respectively arise. At this point the 

 subtropical forests of these ranges are within a short distance of one another. 

 Doubtless for this reason forms elsewhere restricted to one range may in 

 some few instances here be found in both. Further south, these ranges are 

 separated by the increasingly wide Cauca Valley until one reaches the ' knot' 

 of Popayan, but although this attains the altitude of the Subtropical Zone 

 it is lacking in the heavy forests which characterize it and the West Andean 

 Subtropical Fauna is, therefore, not connected here with its Central Andean 

 branch, a fact which presumably accounts for the comparatively small 

 number of West Andean forms found in the Central Andes. 



The distinguishing characteristics of the West Andean Subtropical 



