1896.] NATURAL SCIENCES OF PHILADELPHIA. 31 



The legs may be dark or yellow, or variously marked with these 

 colors, and the proportions of dark and light, though variable, 

 afford good characters within reasonable limits. The anterior tibise 

 are usually yellow in front at least. 



The abdomen differs somewhat in shape, and may be either wholly 

 dark or variously banded or spotted. In every case it should be 

 carefully described, and the color of the ventral surface should also 

 be mentioned. 



The $ genitalia differ in one or two species I have examined, but 

 I have not studied them sufficiently to be able to introduce them 

 into the classification. 



In addition to the above structural and colorational characters, 

 too much stress cannot be laid on the importance of noting the 

 exact localities and the flowers visited. Without the assistance 

 derived from such information, it would have been impossible to 

 unravel the mentzelke series, or satisfactorily arrange the forms 

 allied to affirds. Further, facts of this kind are invaluable in the 

 difficult task of correctly associating the sexes. 



The time of flight should also be carefully noted. Some species 

 are vernal, others (the great majority) fly in late summer and 

 autumn. 



GEOGRAPHICAL AND VERTICAL DISTRIBUTION. 



The species of Perdita are characteristic of the arid region of 

 North America. Of the 70 species, 49 are found in New Mexico, 

 and of these, no less than 34 are in the Mesilla Valley, in the Middle 

 Sonoran (= lower part of Upper Sonoran) zone, at 3,800 feet. 

 Ascending the Valley of the Rio Grande, four species were taken 

 at San Marcial, one at Socorro and nine at Albuquerque, but at none 

 of these places was more than a few day's collecting done. One 

 species was found at San Augustine, on the east side of the Organ 

 Mountains, but has since been observed in the Mesilla Valley. 

 There can be no doubt that Perdita abounds throughout the Upper 

 Sonoran zone in New Mexico. 



At Santa Fe, 7,000 feet, in the transition zone of New Mexico, a 

 good deal of collecting was done in two seasons, but the species of 

 Perdita do not appear to be so numerous as in the Upper Sonoran. 

 Only seven species were taken, although one or two were very 

 numerous in individuals. In the mid-alpine zone no species were 

 seen, either in New Mexico or in the three years residence in Colo- 

 rado. 



