22 Memoirs of the Indian Museum. ' [Voi,. I, 



The third lower molar consists of two laminae, the second of which is poorly devel- 

 oped. An additional mesial rudimentary lamina is present posteriorly, and between 

 the first and second laminae is an additional rudimentary cusp. The posterior mesial 

 rudimentary lamina is very clearly seen in the second molar at a stage at which the 

 third is still simply a flattened plate, which does not hide or interfere with it in 

 any way". 



In the worn tooth of both upper and lower jaw, these traces and rudiments disap- 

 pear, so that there remains simply a flat surface of dentine marked by a sinuous black 

 line, which is all that remains to show the separation of the laminae or tubercular ridges, 

 vide fig. 7. 



Skull. — The distinguishing feature in the skull is that it is an elongated oval with 

 all its lines and ridges flowing in easy curves and with elongated narrow nasals which 

 give to the whole as it were an aquiline expression. The cranium is slightly domed, 

 with a shallow depression separating its contour from the paired eminences that mark 

 the termination of the supraorbital ridges. The lower portion of the anterior edge of the 

 zygomatic plate is vertical, or slants a little upward and forwards to curve backwards 

 to a deep emargination. The lower part of the infraorbital foramen is narrow, with a 

 swelling in front on the maxillary bone. The palatine foramen is equal in length to 

 the upper molar series, and extends posteriorly to just beyond the anterior root of the 

 first molar. The nasals project markedly beyond the gnathion or most projecting 

 point of the premaxilla. The occipital bone is vertical, the occipital crest and the con- 

 dyles being in the same plane. The coronoid suture generally forms a flattened cres- 

 cent, but is very variable. The interparietal is generally oval, bounded by a curved 

 line posteriorly and two curved lines coming to a point anteriorly. 



V aviations .—1 have been rather surprised to flnd that the variation of the nasals 

 is so slight compared with the range of from 49 to 69 per cent, described by Thomas as 

 occurring in Mus nitidus. The extreme range is only from 32"5 to 38-9 per cent. The 

 general average is 35 2 per cent., and in half of the specimens the measurements fall 

 within the limits 34 to 37 per cent., with the remainder almost equally divided above 

 and below the limits given. These figures are based on M. rattus taken as a whole, 

 including all sizes. Thomas in his series of 12 skulls of Mus nitidus from Darjeeling, 

 apparently not collected by himself, found as has been said very wide variations in 

 the length of the nasals. On the other hand I find that the length of the nasal is 

 almost constant, not only in the M. rattus series of skulls but in all the Calcutta 

 species which I have examined, as will be seen from the figures given below. In addi- 

 tion to being constant I find the average measurement is much smaller than given 

 by Thomas, about 36 per cent, of the skull length sinking down to 27 per cent, in 

 Nesokia hengalensis. Possibly the explanation of the discrepancy is that Thomas has 

 reckoned his percentage on the basal length, whereas mine is reckoned on the total 

 length. The following are details giving the maximum above and below the average 

 percentage in the case of each rat, reckoning each size of rattus separately : 

 A^. handicota var. nemorivagus — practically nil ; Mus rattus — 2 -\-2 ; M. rattus var. nitidus 

 ' — 2-f 2 ; N. hengalensis — i-f2'5; M. decumanus — 2-f3"4. Two quite exceptional 



