EXTEBNAL FEATURES 19 



discoloration nor coarsening of the hairs, and this feature 

 enables one to distinguish between adults of the two sexes 

 at a glance. The hairs upon the dorsum of the foot are 

 white. The tail is bicolor, white below and somewhat sooty, 

 or "pepper and salt" above. The hairs are dense and of 

 equal length upon all sides of the tail, and may attain a 

 length of 10 mm. near the base and 30 mm. upon the re- 

 mainder of the tail ; but thej'^ gi'ow in such manner that they 

 lie close to the member upon its dorsal and ventral surfaces, 

 and are spread laterally, so that the tail is somewhat 

 "feathered," inclining in this manner toward the condition 

 obtaining in many arboreal mammals. That this is not an 

 exceedingly ancient specialization is probably indicated by 

 the fact that in juveniles a quarter grown this lateral feather- 

 ing of the tail is only just appreciable and the appendage is 

 densely covered with hairs no longer than 9 mm. 



In all instances juveniles of the genus Neotoma are plumbe- 

 ous in color, as is the case in most rodents of rat- or mouse- 

 like type. 



An examination of large embryos and newly born young, 

 as well as of adults, discloses the fact that there are no areas 

 over which the hair is inclined craniad or in other direction 

 save caudad upon the body, and distad upon the limbs. 



Vibrissae. The mystacial vibrissae of the buccal region^ 

 vary considerably in length, and those measuring in excess 

 of 30 mm. vary in number from 16 to 25. In Homodontomys 

 they may reach a length of 80 mm. and are black. In 

 Neotoma they have white tips, but are usually black at 

 base and grow as long as 65 mm. In Teonoma some are 

 black at base with white tips, and others soHd white, and 

 they may reach 100 mm. in length. Each grows from a sub- 

 cutaneous, tactile papilla springing from the tough, fibrous 

 tissue beneath the integument of the upper lip and is served 



'For vibrissal terminology see Pocock, 1914. 



