158 Bulletin American Museum of Natural History. [Vol. XXXIV, 
fine differences been recognized, as indeed many of them were, and ignored. 
The ‘splitting’ craze of to-day may not be so harmful to scientific progress 
as was the ‘lumping’ craze of thirty years ago, but it is certainly burdening 
nomenclature to an embarrassing and unprofitable degree. As the field 
of discovery in the matter of noteworthy new forms is becoming exhausted, 
as in North America, intergrades between already recognized slightly differ- 
entiated forms are given names and appear in faunal lists, to the confusion 
of even the expert unless he be provided with plenty of topotype material. 
While faunistic studies cannot be made too intensive, minor local differen- 
tiations do not require pigeon-holing by means of nomenclature; they can 
be otherwise recorded as interesting facts in environmental differentiation. 
GENERAL CONSIDERATIONS. 
Before taking up the subject of genera and subgenera in relation to the 
American forms of tree squirrels heretofore usually referred to the genus 
Scvurus, it seems desirable to present certain general facts that have a bear- 
ing on genetic relationships, and also to discuss some other generalities that 
may be better considered here than elsewhere. 
Premolar formula. 
In the Sciurinee the presence or absence of p? is usually looked upon as 
of small taxonomic importance, since in some groups it is absent or reduced 
almost to disappearance, although in other groups it not only reaches the 
crown level of the molars but is sometimes an obviously functional tooth. 
Formerly it was sometimes looked upon as a feature of the milk dentition, 
which might or might not reappear in the permanent dentition, and there- 
fore in some cases was evanescent. On the contrary, as stated by Nelson 
in 1899 (/. c., p. 49): “It is not present with the milk premolar in immature 
skulls, but appears coincidently with the permanent premolar and is equally 
persistent.” Geographically considered, it is absent in all species of South 
American squirrels except the peculiar and highly specialized genus Micro- 
scvurus of the Andean highlands of western South America, which occurs 
also as an intrusive genus in Central America as far north as central Costa 
Rica, and in the still more specialized genus Sciurillus of the Guianas. It 
is present in all species of North American squirrels except in Parasciurus 
(Parasciurus + Arcosciurus) of eastern United States and the Mexican 
tableland, and the intrusive Mesosciurus ( = Guerlinguetus, part, auct.), of 
