Method of talcing the Incisive Index in Rodents. 289 



rather prominent postorbital processes which are characteristic 

 of I he larger species. 



Measurements. — Hind foot measured on the skin about 00 mm. 



Measurements of skull (with corresponding dimensions of a skull of 

 ansorgei in parentheses). — Condylo-basal length 68"7 (77) mm. : zygomatic 

 breadth 33 ca. (38) ; interorbital constriction 10'2 (11 '4) ; mastoid breadth 

 242 (27-1) ; nasal 28 x 10-1 (33 X 10-5) ; incisor to m. 3 388 (42*3) ; dia- 

 stema 22 (23'9) ; length of anterior palatal foramina 8"8 (82) ; length of 

 preorbital fossa for masSeter medialis 14 (14) ; least diameter of zygo- 

 matic plate 6 - 2 (8"1) ; molar crowns 10'5 (12'8). 



XXVIII. — The Method of taking the Incisive Index in 

 Rodents. By Oldfield Thomas. 



(Published by permission of the Trustees of the British Museum.) 



The angle at which rodent incisors protrude from the jaw 

 has long been recognized as an important character of diffe- 

 rent groups, but it is only recently that an attempt has been 

 made to define that angle more exactly, instead of merely 

 speaking of "incisors thrown forward" and so on. 



Diagram showing method of taking incisive index in rodents, 



This angle, when exactly measured and defined, proves 

 to be exceedingly useful as a systematic character, but it is 

 essential that the way it is taken should be clearly understood, 



