jyn] Taijlor: Mammals of the 1909 Nevada Expedition. 247 



The examination of these four skulls shows the number of the 

 distinguishing cranial characters to be considerably less than at 

 first supposed. They show a very decided approach to desertorum. 

 this being what might be expected to occur. It is possible that 

 the disposition of the form nevadensis as a subspecies of desert- 

 orum would more correctly represent the status of the Nevada 

 wood rat; but this is hardly justifiable without an examination of 

 more material, including skins. 



There is now available in the ^luseum collection a series of 

 Neotoma desertorum from the Colorado River, collected on the 

 California side l)y the Colorado River Expedition (^lus. Vert. 

 Zool.) of 1910. This series contains animals of various ages, 

 and a comparison with the topotype series of nevadensis brings 

 to light the following facts. 



Specimens absolutely comparable, as regards age, were 

 selected. Characters used in determining age were largely 

 cranial, the most important being the degree of development, 

 emergence and wear of the teeth. 



The series as a whole presents characters indicating the dis- 

 tinctness of nevadensis. The posteriorly extending tongues of 

 the premaxillaries, for example, are narrower than in nevadensis, 

 and the zygomatic arches are more widely spreading. The rostra 

 of the series of desertorum from the Colorado River are shorter 

 and broader than in nevadensis. The anterior roots of the zygo- 

 matic arches are on the average wider in the series of desertorum ; 

 the hanmlar processes are more curved, on the average, and the 

 incisors show differences as outlined in the description of neva- 

 densis, being longer and less curved in desertorum. Only one of 

 the comparable series of desertorum (no. 10424) has the bay in 

 the dorsal contour of the foramen magnum as sharply defined 

 as in nevadensis. While there is no appreciable difference in 

 weight in the mandibles of the two series, and although it is 

 impossible to say, as a result of this comparison, that the j^osterior 

 roots of the zygomatic arches are different, still the majority of 

 skull characters clearly points to the distinctness of the Nevada 

 wood rat. 



