201 Mr. O. Thomas — .4 revised 



III ]Mr. Wroughton's admirable monograph of Oiomys*, 

 the characters used are almost entirely dental, little attention 

 being paid to the skull. Now, liowever, 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 laminje, 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 O. brantsii and 0. uin- 

 sulcatus, I have thought it better only to consider those 

 represented by 0. anchietee and laminatus as subgenera of 

 Otomys, their distinction being almost entirely based on the 

 plastic dental characters. And the same M'ith Farotomys 

 brantsii and littledalei. 



A. Nasals not excessively expanded ante- 



riorly. Tendency to grooving of incisors 



and extra lamination of molars less ; 



lower incisors not or very faintly grooved; 



»«' with 4 or, at most, 5 laminse. 

 a. Bullte very large. No special nasal 

 broadening. M^ composed of two com- 

 plete laminpe and a modified posterior 

 portion 1 . Payotomys, g. n. 



«'. Upper incisors grooved 1 «. Pdrotomys, s. s. 



b^. Upper incieors smooth 16. Liotomys, subg. n. 



6. Bullaj normal. A slight nasal broaden- 

 ing. M^ composed of three complete 

 laoiinoe and a posterior trefoil 2. Myotomys, g. n. 



B. Nasals excessively broadened anteriorly, 



the preniaxillaa outside them not or 

 scarcely visible 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^ with 6 laminae or raoref 3. Otomys. 



c. M\ composed of 4 iaiuiuae .3 «. Otomys, s. s. 



d. Ml with more than 4 laminae. 



c*. Ml with 5 lamiuie, m^ with 7 3 6. Anchototnys, subg. n. 



d'. Ml with 6-7 laminae, m^ with 9-10 . 3 c, Lamotomys, subg. n. 



» Ann. & Mag. N. H. (7; xviii. p. 264 (1906). See also Dollman's 

 paper on the East African forms, op. cit. (8) xv. p. 149 (1915). 

 t Five in O. dtnti. 



I 



