604 Bulletin American Museum of Natural History. [Vol. XIX, 



which species the skull closely agrees in general form; total length, 36; 

 greatest mastoid breadth, 23; length of nasal, 13. As regards size, 

 out of 40 adults only 4 exceed a total length of 240 (maximum, 250), 

 and only 6 have a tail length of 136 or above (maximum, 140). Even 

 the maximum size falls far below tne minimum for adults of P. agilis. 



Represented by 84 specimens, collected as follows; Rosario, 28, 

 Jan. 24-27; Mt. San Gabriel, 2, Jan. 28; Rio del Bocas, 13, Feb ; 8-9; 

 Villa Ocampo, 5, Feb. 13; Rancho Santuario, i, Feb. 22; Rio Sestin 

 35, April 9-17. 



Twenty adults (12 males and 8 females), from Rio Sestin, measure 

 as follows: 12 males, total length, 234 (223-244); head and body, 

 101.7 (98-111); tail vertebrae, 131 (121-136, only 2 below 130); hind 

 foot (without claws), 32.4 (32-33); ear from notch, 13.2 (13-14): 8 

 females, 225.5 (218-238); 101.3 (98-105); 123.4 (120-133); 3 2 -5 

 (32-33); 13.7 (13-14)- 



Nine males from Rosario: Total length, 229.6 (222-232, with i addi- 

 tional 250); head and body, 103 (95 114, with only i above 105); tail 

 vertebras, 126.7 (121-136, with only i above 130); hind foot, 34.6 



(34-35); ear > I 3- 1 (iS-^)- 



Eleven specimens (6 males and 5 females), from Rio del Bocas: 6 

 males, total length, 232.5 (223-245); head and body, 103.7 ( I02 - I0 5); 

 tail vertebrae, 129 (121-140); hind foot, 34.5 (32-36); ear, 13.5 (1314); 

 5 females, 230 (222-245); I0 3- 2 ( I02 - I0 5)'> I2 7 (120-140, only i 

 above 134); 33.8 (32-36); 13.5 (13-14). 



Perodipus obscurus is the darkest -colored species of the genus 

 thus far known, except possibly O. agilis, which has, however, 

 a very different coloration, and is nearly double the size of the 

 present species, which about equals O. ordi and O. chapmani. 

 From all the smaller members of the group it differs too radi- 

 cally in coloration to require special comparison. 



19. Perognathus flavus Baird. 



Six specimens: Rancho Santuario, i, Feb. 21; Rio Sestin, 

 5, April 16 and 17. 



20. Perognathus nelsoni Merriam. 



Ten specimens, as follows: San Gabriel, 2, Feb. 19 and 20; 

 Rancho Santuario, 6, Feb. 22-March 8 ; Rio Sestin, 2, April n. 



Seven adult males measure: Total length, 178 (162-190); 

 head and body, 82.4 (76-89); tail vertebra?, 99 (95-102); 



