MURID^— SIGMODONTES— HESPEROMYS LEUCOPUS. 



75 



the specimens from Cape Saint Lucas in the nakedness of the tail and extension 

 of the dusky on (he metatarsus — the latter being one of the essential features 

 of the species as compared with leucopus. In length of tail alone, these speci- 

 mens grade into the "gambeli" with which they are associated; yet, the 

 other features being perfectly tangible and distinctive, we shall refer the 

 specimens in question to aztecus, with no reasonable doubt of the correctness 

 of so doing. 



As Mr. Xantus collected extensively on the west coast of Mexico, as well 

 as in Lower California, it is possible that these examples of aztecus have been 

 accidentally mixed with his Saint Lucas collections. 



The following table gives the measurements of a part of the specimens 

 referable to leucopus ("gambeli") with measurements of some of them. 

 Those representing aztecus will be found under the head of the latter. 



Table XVII. — List of specimens of H. leucopus (" gambeli") from Cape Saint Lucas, collected by J. Xantus, 



with measurements. 



Having exhausted the data at our disposal respecting the nominal species 

 of Hesperomys (except "cognatus", noticed below) that are referable to 

 leucopus, we are prepared to bring out three geographical races or varieties 

 that are distinct enough, we think, in their strongest development at 

 least, to merit varietal names, although (we wish it distinctly understood) 

 they pass insensibly into ordinary leucopus. These three are gossypinus, 

 sonoriensis, and eremicus. The first of these is nearest leucopus, and is hard 

 to define satisfactorily. The second is better marked, and, in its extreme, 

 readily discriminated from leucopus, though comparison of some hundreds of 

 specimens show a perfect transition into the latter. The third is strongly 

 marked as a local race, and might be held, with much show of reason, as a 

 good species, as, in tact, we considered it until we saw intermediate specimens. 



