201 Mr. 0. Thomas — A revised 



In Mr. Wroughton's admirable monograph of Otomys *, 

 the characters used are almost entirely dental, little attention 

 heing paid to the skull. Now, however, taking cranial 

 characters into full consideration, I find that the group 

 appears to be divisable into three genera, as shown below. 



Although not easily defined in a key, the general shape 

 of the skull is quite distinctive of the three genera, and is, 

 I consider, the best indication of their relationships. On 

 the other hand, the grooves on the incisors, and the numbers 

 of the molar laminae, used so effectively by Wroughton and 

 Dollman for the sorting of the species, are so plastic, and 

 show so wide a range of variation, that, however useful for 

 specific distinction, they have to be used with great caution 

 when generic divisions are in question. 



On this account, while distinguishing as full genera the 

 obviously natural groups typified by 0. brantsii and O. uni- 

 su/catus, I have thought it better only to consider those 

 represented by O. anchieta? and Jaminatus as subgenera of 

 Otomys, their distinction being almost entirely based on the 

 plastic dental characters. And the same with Parotomys 

 brantsii and littledalei. 



A. Nasals not excessively expanded ante- 



riorly. Tendency to grooving of incisors 

 and extra lamination of molars less ; 

 lower incisors not orrery faintly grooved; 

 ?>t 3 with 4 or, at most, 5 laminae. 



a. Bullae very large. No special nasal 



broadening. M 3 composed of two com- 

 plete laminae and a modified posterior 

 portion 1. Parotomys, g. n. 



a 1 . Upper incisors grooved la. Parotomys, s. s. 



b 2 . Upper incisors smooth 16. Liotomys, eubg. n. 



b. Bullae normal. A slight nasal broaden- 



ing. M 3 composed of three complete 



laminae and a posterior trefoil 2. My otomys, g. n. 



B. Nasals excessively broadened anteriorly, 



the premaxillae outside them not or 

 scarcely risible from above. Tendency 

 to grooving of incisors and extra lamina- 

 tion of molars at a maximum ; lower 

 incisors, as well as upper, deeply grooved ; 

 m 3 with 6 laminae or moref 3. Otomys. 



c. Mi composed of 4 laminae 3 a. Otomys, s. s. 



d. Mi with more than 4 laminae. 



c 2 . Mi with 5 laminae, m s with 7 3 b. Anchotomys, subg. n. 



d 2 . Mi with 6-7 laminae, m 3 with 9-10 . 3 c. Lamotomys, subg. n. 



* Ann. & Mag. N. H. (7) xviii. p. 204 (1906). See also Dollman 's 

 paper on the East African forms, op. tit. (8) xr. p. 149 (1915). 

 t Fire in O. dtnti. 



