1915. ] 



INTRODUCTION. 



17 



of the maxilla and the jugal, but this changes considerably with age 

 and is not much used in diagnoses. The pterygoids have a wide 

 range of variation an(J furnish excellent characters in many species. 

 The large and concave pterygoids of T. hulhivorus have been used as 

 the main basis for a subgenus ( Megascapheus) ,^ but while unique they 

 certainly do not carry more than specific distinction. The general 

 variation is in elevation or depression of the wings of the pterygoids, 

 wide or narrow, and sharply V-shaped or widely U-shaped fossa. 

 The audital bullae vary greatly in relative size and form, the largest 

 being smoothly rounded and globose, the smaller more elongated 

 and angular. The basioccipital varies greatly in relative length and 

 width, generally becoming narrower as the bullae increase in size and 

 crowd upon its margins. Many sHght detailed differences can also be 

 used to advantage as characters. Many excellent characters in the 

 lower mandibles are not generally used in diagnoses, as in most forms 

 there are others more convenient. 



DENTITION. 



The dental armature of Tkomomys is included and fully discussed 

 with the other genera in Dr. Merriam's revision of the Geomyidae.^ 

 Throughout the genus Thomomys there is surprising uniformity of 

 tooth form and enamel pattern in the molariform teeth; while 

 many minute variations furnish distinctive specific characters, the 

 necessity of studying them with a lens renders them inconvenient 

 for general use, and they rarely serve to separate closely related 

 forms. On the contrary, the incisors furnish conspicuous group as 

 well as specific characters. The angle of the upper incisors to the axis 

 of the skull varies from the very protruding incisors of T. hulhivorus, 

 T. leucodon, and T. umbrinus to the abruptly incurved incisors of T. 

 pygmseus, T. idahoensis, and T. ocius. A narrow groove near the 

 inner edge of each upper incisor is deepest and most conspicuous in 

 the monticola group, but so slight and obscure in other groups as to 

 be detected only by critical examination with a lens; it is usually 

 most obscure in species with projecting upper incisors. In some 

 species the incisors are rather slender, and in others relatively stout. 



SEXUAL VARIATION. 



The male in most species is considerably larger than the female and 

 its skull is heavier, more angular, and more heavily ridged in old age. 

 This sexual difference is greatest in the hottde group and apparently 

 least in the monticola smdfuscus groups, but is noticeable in all. 



The functional mammae vary in fully adult females of different 

 species from 3 to 7 pairs, usually arranged in 2 pairs of inguinal, 2 or 



1 Elliot, Field Columb. Mus., zool. ser. Ill, 190, 1903. Type, Diplostoma bulbivorum Richardson. 

 2N. Am. Fauna No. 8, 69-108, 1895. 



98121°— 15 2 



