2-46 University of California Publications in Zoology [Vol. 12 



In addition, the Museum contains four skins (nos. 5636, 5637, 5825, 

 5826) taken in 1861 by Dr. J. G. Cooper at Fort Mohave, on the 

 Arizona side twelve miles above Needles. 



It is at once patent that of the five distinct species of Perognathus 

 found along the lower Colorado, P. penicillatus is both most abundant 

 and most widespread. Very many more individuals were trapped day 

 by day than were preserved. 



It may be asked why explicit record is not here presented, not 

 only of specimens saved, but of all the individuals trapped. The reason 

 is that so similar in external appearance are the various species of 

 Perognathus, that especially with our previous unfamiliarity with 

 them, we were liable to many mistakes in identification in the field. 

 After museum study was made of the suites of specimens brought home, 

 with skulls cleaned and measurements tabulated, it was found that 

 errors had actually occurred in a number of cases thus checked up. 

 Perognathus penicillatus and P. intermedins had been confused on 

 the Arizona side, and P. i>< nicillatus and P. formosus on the California 

 side, especially in the cases of immatures. Field determinations w T ere 

 therefore totally disregarded. 



It may scarcely be necessary to remark that penicillatus (plus 

 angustirostris, if the form designated by the latter name be consid- 

 ered tenable), intermedins, spinatus, and of course formosus, are all 

 distinguishable with absolute certainty (for characters see Osgood, 

 1900). No difficulty was experienced in allocating all specimens as 

 soon as cleaned skulls were available. 



Although obtained at all collecting stations and from all associa- 

 tions, reference to the graph for frequency of occurrence (see fig. D) 

 clearly shows that Perognathus penicillatus has marked preferences for 

 one particular environment. This preferred habitat is characterized 

 by ground of fine-grained sand. The second bottom along the river 

 and the broad washes of the flatter parts of the desert furnish the 

 ideal conditions; and in the most typical of these penicillatus is either 

 the exclusive or the prevailing member of the genus present. 



Penicillatus is the only species invading alkali depressions at the 

 edges of the broad valleys back from the river (where Suaeda grows), 

 and is the only species occurring regularly on parts of the overflow 

 area of the river bottom. Referring again to the frequency record 

 (fig. D), it is shown that penicillatus was found in every member of 

 the riparian group of associations, even into the willows. Individuals 

 were always, however, as far as record and memory serves to estab- 



