46 BULLETIN 61^ UNITED STATES NATIONAL MUSEUM. 



tions is that of Doctor Coues (1875, 613), who states that according 

 to his observations the species in Arizona is not "specially addicted to 

 the water, frequenting dry herbage and bushes." This statement may 

 justly be withheld as evidence, however, for, as will be shown later, 

 the identity of the species which Coues referred to is doubtful. Since 

 Cope's Duck Creek, New Mexico, specimen was also found in swampy 

 ground in the vicinity of water, we may conclude that as far as our 

 present knowledge goes megalops throughout its range prefers the 

 vicinity of water. If this is true, while of more general distribution 

 in the southern parts of its range (States of Mexico, Puebla, etc.), to 

 the northward it probably becomes confined to those habitats in 

 which water is to be found for the greater part of the year. 



Range. — Much. of the unity of this form may be accounted for by 

 the character of the region which it inhabits. Geographicall}' the 

 Mexican plateau is a tableland lying at an elevation ol» 3,000 to 

 8,000 feet, and separated from the Gulf of Mexico on the east, and 

 the Gulf of California and the Pacific Ocean on the west, by parallel 

 mountain chains and a narrow strip of coastal plain. To the north 

 it descends gently into the open basins of southern Arizona and 

 southwestern New Mexico, being limited in its northward extension 

 by the escarpment of the Colorado plateau, thus including the 

 Proplateau region of southern United States (see Ruthven, 1907). 

 Southward it extends to the isthmus of Tehauntepec, where it ends 

 by the junction of the two parietal mountain ranges. This plateau 

 has been built of the accumulated waste of the preexisting mountains, 

 and volcanic discharges, the existing mountain peaks protruding 

 through this mass, mainly in the higher parts of the plateau, as 

 nunataks in an ice field (Heilprin, 1902, 774-775). 



The climate of the plateau may be characterized in general as tem- 

 perate (Heilprin, 1900, 777). The amount of precipitation is vari- 

 able. In southern Mexico it is comparatively great, the rainy season 

 lasting about six months, and the streams and lakes are perpetual. 

 To the northward, however, owing to the influence of the eastern 

 chain of the vSierra Madres, much of the moisture carried by the 

 warm gulf winds is precipitated on the coastal plain, so that in their 

 subsequent passage over the tableland they are desiccating winds. 

 During the short rainy season of three months the flora of this 

 arid region grows luxuriantly, but during the long dry season, in 

 which the region is exposed to the glare of the tropical sun and the 

 hot, dry winds, the vegetation becomes parched, and the streams and 

 lakes partially or entirely dry. The aridity of this area increases to 

 the northward, from San Luis Potosi northward being known as the 

 Chihuahuan desert that grades into the Proplateau region of south- 

 western United States. This entire region, therefore (northern part 

 of the Mexican plateau and the Proplateau regions), is arid. The 



