LHjyodomi/.s pldllipsi Gray. 87 



Ajiisco begin to form the mountain rim on that side of the basin. Lying 

 just west of Tialpam is a great lava bed, Icnown as the Pedregal, wliicli 

 descends from tiie nortliern base of the pealv of Ajusco and reaches 

 down the slope and out along the southwestern border of the valley - 

 The peak of Ajusco, rising to an altitude of 12,600 feet, lies about 9 miles 

 in a southwesterly direction from Tialpam. Commencing near the eastern 

 base of this peak and following down the eastern border of the lava bed 

 to the bottom of the valley at Tialpam is a bed of fine, grayish volcanic 

 ashes or sand. Originally this deposit of sand did not reach the bottom 

 of tlie valley, but the heavy sunmier ruins gradually washed it down 

 until it is now spread for a mile or two out over the bottom, at the base 

 of the hills extending east from Tialpam. In this bed of fine, sandy 

 soil, beginning almost with the last houses of the town, these kangaroo 

 rats are very abundant. The ground occupied by them at Ajusco, Huit- 

 zilac, and Amecameca was once covered Avith pine timber, but is now 

 used as corn or grain fields, and it is altogether likely that the distribu- 

 tion of the species in all of tliese localities from the border of the Valley 

 of Mexico followed the subjection of the land to cultivation. At the 

 jjresent time, although there are large areas of apparently suitable land, 

 its distribution is local and restricted to com2:»aratively small districts." 



At a later date Mr. Nelson found Dipodoiays pliilUp.u on the 

 plains of Puebla and thence northerly into eastern Tlaxcalaand 

 western Vera Cruz. Concerning its occurrence in tliis region he 

 Av rites : 



"The species was not encountered again until I reached the stale of 

 Tlaxcala. Thei'c, in the eastern half of the state, about the northern and 

 eastern base of the Ccrro de Malinche and the towns of Huamantla and 

 San Marcos, it is common. Extending thence easterly along the same 

 sandy plain into the state of Puebla, it is also very numerous. 



" In this latter state the species reaches its extreme southeastern limit 

 near the towns of Canada ]\Iorelos and Esperanza. To the north of these 

 localities it is abundant about the towns of San Adres, Chalchiconmla, 

 and up the adjacent western base of Mount Orizaba to an altitude of 9,000 

 feet. From these points it ranges across to Ojo de Agua and San Juan de 

 los Llanos, in the same state. To the northeast its extreme limit is found 

 about the northern and western base of the Cofre de Perote, a little east 

 of tlie town of Perote, m the state of Vera Cruz. Tiiis gives the species 

 a known range over jiarts of four states. It occu[)ies the extreme soutliern 

 end of tiie Mexican table-lands, and is therefore the extreme southern 

 rejjresentative of the genus. 



" So far as my observations have extended, this species is confined strictly 

 to the areas of volcanic sand produced by two volcanic centers : first, the 

 sandy belt lying between Mount Popocatapetl and the Cerro de Ajusco, 

 some thirty miles in a westerly direction from the first-named peak, and 

 all within the State of Mexico ; vecoiid, the much larger district having its 



