1915.] Allen, Review of the South American Sciuride. 159 
South America, which ranges north in Central America as far as northern 
Nicaragua. These facts seem to imply genetic significance in the presence 
or absence of p*. 
The degree of development of p? varies in different super-specific groups. 
In Tamiasciurus it is reduced to a minimum and is often absent. In this 
group it is truly vestigial, being always very small, barely or not always 
piercing the gum, and nearly hidden beneath the crown of p‘. Yet its 
geographic constancy is noteworthy, it being present in about equal devel- 
opment in all three species of the group, and in all of their numerous sub- 
species, which collectively occupy all the wooded portions of North America 
from northern Georgia and the mountains of northern Lower California 
north to the limit of trees. In Neoscturus and Echinosciurus it is generally 
small and slender, but usually reaches the crown level of p*. In Hespero- 
scvurus, Otoscvurus, and in the tropical Microsciurus and Syntheosciurus, 
it is a conspicuous and functional tooth, frequently with a bicuspid crown. 
Its presence and degree of development, or absence, usually accompany 
marked specialization in other features. 
Mamme. 
Text Figures 13-16, p. 165. 
All American tree squirrels have either 6 or 8 (either 3 or.4 pairs) of 
mamme, those with 6 having one of the abdominal pairs absent. All 
North American species have 8 mamme, except Barosciurus of eastern and 
southern Mexico, Guatemala and Nicaragua, and the intrusive South Amer- 
ican Mesosciurus. All the squirrels of the northern border of northern 
South America (north of the llanos of Venezuela) and of the Andean region 
south to Peru and Bolivia, have only 6 mamme, while all other South Ameri- 
can squirrels have 8 mamme, including not only the giant squirrels of the 
Amazonian drainage but also the little guerlinguets of the Guianas and 
eastern Brazil. Furthermore, the South American species with 8 mammee 
are widely separated geographically from the North American species 
with 8 mamme. ‘The species with 6 mamme include also species of large 
size as well as the smallest American forms. ‘Thus the number of mammee 
is not fortuitous but a factor of great constancy, as it is also one of high phy- 
siological significance. It doubtless has considerable taxonomic value, 
since supernumerary mamme are about as rare as supernumerary teeth. 
