792 JOURNAL, BOMBAY NATUttAL HISTORY SOCIETY, Vol. XIV. 



Tail. — The tail is coarsely and distinctly annulated. At middle there are 

 about eight rings to the centimetre. To each scale there are from one to 

 three stiff hairs varying in length from 1 to If millimetres. 



MammcB. — There are 10 mammse, 4 pectoral and 6 inguinal. 



Feet. — The feet are in no way peculiar. Soles naked, with the usual six 

 tubercles. 



Ears. — Ears as in the related species. 



Skull. — In a general way the skull of Mus burrus rather closely resembles 

 that of Mus nortegicus. It is slightly smaller than that of the house rat, 

 however, the dorsal profile is less convex, the rostrum shorter and broader, 

 the interorbital region less constricted, the zygomatic arches less flaring, the 

 braincase broader and less elevated, the interpterygoid space wider, and the 

 audital bullaa slightly different in form and more resembling those of Mus 

 ratius. Nasal bones gradually narrowed from front to back, their outer 

 margins nearly straight except for the usual slight downward curve. Pos- 

 terior portion of mandible somewhat deeper than in Mus norvegicus. 



Teeth. — The teeth are relatively somewhat larger than in the house rat, 

 but the enamel pattern shows no peculiarities. 



Measurements. — External measurements of type : Total length, 430 mm. ; 

 head and body, 215 ; tail, 215 ; hind foot, 43 ; ear from meatus, 24 ; ear from 

 crown, 18; width of ear, 17. Average of ten specimens from the type locality: 

 Total length, 407 (385—430); head and body, 205 (195— 225); tail vertebrae, 202 

 (193—215); hind foot, 42-5 (42—43); hind foot, without claws, 40*6 (40—41-4). 



Cranial measurements of type : Greatest length, 47*6 mm. ; basal length, 

 43; basilar length, 40; palatal length, 23'4 ; least width of palate between 

 anterior molars, 5'4 ; diastema, 14 ; length of incisive foramen, 8*4 ; combined 

 breadth of incisive foramina, 2-8 ; length of nasals, 17'4 ; combined breadth 

 of nasals, 5 ; zygomatic breadth, 22 ; interorbital breadth, 8 ; mastoid breadth, 

 17'4 ; breadth of braincase above roots of zygomata, 17 ; depth of braincase 

 at anterior border of basioccipital, 12*2 ; fronto-palatal depth at posterior 

 extremity of nasals, 11*8 ; least depth of rostrum immediately behind incisors, 

 9 ; mandible, 28 ; maxillary toothrow (alveoli), 7'6 ; mandibular toothrow 

 (alveoli). 7*4. 



Specimens examined.— Twelve, all from Trinkut Island, Nicobars. 



Remarhs. — In external characters, the series of 12 skins show practi- 

 cally no variation. The younger specimens are less clear buff beneath, but 

 with this exception, the colour is perfectly constant. The skulls are likewise 

 very uniform. Here the variation is confined to the slight differences in size 

 usually correlated with greater or less age. The nearly straight lateral outline 

 of the nasal is an unvarying characteristic of the entire series of skulls. 



Whether Mus burrus is a direct descendant of the house rat cannot be 

 decided, but its relationship with this wandering species is not remote. The 

 dark colour and less spreading zygomata are alone sufficient to distinguish the 

 Nicobarian animal, however, from any of the very numerous specimens of 

 Mus norvegicus thai have come under my observation. 



