300 Bulletin American Museum of Natural History. [Vol. XXXIV, 
(see antea, p.190). The only other known representatives of this sub-family 
occur in the Malay Archipelago and in West Africa. Its presence on the 
east coast of South America therefore may be taken as possibly another 
indication of former land connection between South America and Africa. 
The genera will now be briefly reviewed with reference to their constitu- 
ent groups and their distribution and interrelationships. 
Mvcrosciurus is represented by nearly 20 rather closely allied forms, most 
of them rated as species. They are all very small, the difference in size 
between the largest and smallest being but little greater than the range 
fos 7 
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Csr Aptis) cf Pa REBTO 
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ine Siw Hee ° . ore 
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e Ome oe 
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° 
sp esanie ots 
4 . 
a CH ch 
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UG e* *, 
e Se cigie 
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° 
Fig. 24. 1, Hadrosciurus; 2, Urosciurus; 3, Simosctiurus. 
of individual variation in a good series of almost any of the species. In 
coloration and in the character of the pelage there is little diversity, and in 
the case of the latter the difference is correlated with sea level and high alti- 
tude environment. It differs widely from all other generic groups of Ameri- 
can squirrels, and is the only South American genus (Sciurillus excepted) 
with two upper premolars. Its peculiar geographic distribution has already 
been stated. It may be considered as an old generalized type. 
Mesosciurus has the widest distribution of any of the South American 
genera, it having representatives in Central America as far north as Costa 
