SQUIRRELS OF MEXICO AND CENTRAL AMERICA 23 



panied by increased intensity of coloration. This is well shown 

 by the dark, rich colors of Scinriis aiircogaster hypofyrrhus of 

 the humid tropics compared with the paler colors of the closely 

 related S. atireogaster, of the arid tropics a little farther north. 

 A similar intensitication of color is characteristic of the squir- 

 rels inhabiting the humid east coast from Vera Cruz to Pan- 

 ama, as contrasted with the paler species of the more arid west 

 coast from Mazatlan to Costa Rica. 



SUBGENERA OF NORTH AMERICAN SQUIRRELS. 



The tree squirrels of North America may be separated into ten 

 groups, distinguished by cranial, and sometimes by external 

 characters. Six of these groups have been already recognized 

 as of subgeneric rank, and four others are here named. 



In discriminating the subgenera of squirrels one of the charac- 

 ters on which much stress has been laid is the presence or absence 

 of the small upper premolar. The discussion of the groups in 

 detail is prefaced, therefore, with a few remarks on this tooth. 

 Trouessart makes the erroneous statement that in the subgenus 

 Macroxiis the upper molar series is often | in the young and |- 

 in the adult, adding that the rudimentary premolar is more or 

 less speedily shed.^ Under Macroxus he names six Tropical 

 American squirrels, viz., aureogaster, cBstuaiis, hoffmanni, 

 deppei, chry stints, and pusilliis. Of these asttians and ho_ff- 

 7nanm never have the extra premolar, while all of the others, 

 with the possible exception of chrysiirus, always have it in the 

 adult skull. The six species named above represent four sub- 

 genera, two of which were recognized by Trouessart in the same 

 paper ; one has since been separated by Allen ; another is de- 

 scribed below. In all North American Squirrels having the extra 

 premolar, except the Chickarees or Red Squirrels of the United 

 States and Canada (subgenus Tamtascitirus) , it is well devel- 

 oped and permanent. Among the Chickarees it is extremely 

 small, variable in size, shape, and position, and frequently ab- 

 sent. Dr. Allen states, in his recent revision, that it is absent 



' Le Naturaliste, II, No. 37, pp. 292, Oct., iSSo. 



