JESimpson, 1938. 68. 270ft., Rancho de Costa Rica, Rio Sonora, Sonora. May 3, 1938, continued. and 3 ♀♂; not saved), and a juvenile Peromyscus eremicus → Perognathus penicillatus (dead in trap), (not saved); in live traps: 7 Reithrodontomys sp. and 5 Perognathus flavus sonoricensis (topotypes). Put the latter five: ♂291-294; ♀296; and the five pocket-mice saved alive from yesterday: ♂286-87, ♀288-290; of these ten: ♀288, 289, and 296 were molting. Dr. Benson suspected the Reithrodontomys sp. are an entirely new species; so we have taken good series. Probably the 3 harvest mice(260-262) I caught 11.3 miles w. of Hermosillo are the same form. Here at Costa Rica the live-traps have come into their own — for the most harvest mice and pocket- mice (P.f. sonoricensis) that were especially wanted were taken in the tin-boxes and not in the mousetraps. So, previously stated to the contrary, I must admit that under particular circumstances the tin-boxes are probably more effective in taking small mice than the "museum special" mousetraps. Dr. Benson in setting his mousetraps at the edge of the field caught many Perognathus penicillatus near the mesquite in sandy soil and many P. baileyi near irrigation ditch at mesquite border in harder ground while the traps out in the wheat-field and cornfield caught few of those pocket-mice but many P. flavus. Aside from pocket-mice, habitat restriction was also seen in that few Peromyscus eremicus were