1907.] Dr. Hossack: The Rats of Calcutta. 21 



verse laminae overlapping each other obliquely from before backwards. The essential 

 difference between these laminae and those found in the genus Nesokia is that the 

 former are much more sinuous and tend to break up into definite cusps, three in the 

 upper molars (a large central anterior one, flanked by a smaller one on each side 

 which is slightly posterior) and two only in the lower molars. This sinuous arrange- 

 ment of the cusps is well shown in the figure of the unworn tooth of Mus 

 rattus (plate i^ fig. 10), while the sharp way in which the cusp stands out from the 

 lamina is better displayed in the figure of the unworn tooth of Mus decumanus 

 (fig- 25). 



The upper incisors are orange in colour, with a comparatively flattened anterior 

 surface which shows fine longitudinal parallel markings just as in Nesokia^ though 

 Thomas and Blanford make the incisors smooth as against the " sculptured " surface 

 seen in the same teeth in Nesokia. The pair of lower incisors are much paler, rather 

 a faint yellow-ochre than orange, and at times may be white. The upper molars 

 show the following structure : — 



The ist molar consists of three tricuspid laminae as above described, the tubercles 

 or cusps forming three longitudinal rows. 



The 2nd molar consists of three laminae, of which only the middle one is fully 

 and typically developed. The anterior one is only rudimentary and is represented by 

 the inside cusp. The posterior one is also not fully developed and is represented 

 mainly by the central tubercle, and less distinctly by the outside cusp, the inner being 

 altogether wanting. 



The 3rd molar is like the second, but the middle lamina is less distinctly marked 

 off into three cusps or tubercles, while the posterior one is very small and shows only 

 the central tubercle. 



I have had to go into this question rather in detail, as in some specimens the laminar 

 division of the tooth is more distinctly marked than the tricuspid or tritubercular 

 division of the lamina. The main characteristic, however, of the fully developed unworn 

 tooth is the division of the lamina into distinct tubercles or cusps, though it must 

 be admitted that in the early development of the tooth the lamina appears with 

 very little sign of subdivision, approximating to the condition typical of the Nesokian 

 tooth. 



The laminae in the lower molars are divided into two tubercles, giving a double 

 row of cusps separated by a median furrow. The first molar consists of three of those 

 laminae, with a rudimentary fourth one behind mesially, vide fig. 12a. 



In addition it may show an extra cusp between the second and third laminae on 

 the outside of the tooth, vide fig. 12b. 



It is noteworthy that additional laminae or cusps in the upper molars are found 

 on the inside, whereas in the lower they are on the outside. The second molar consists 

 of two fully developed laminae, with a median rudimentary one behind. In addition 

 there are found on the outside the following additional rudiments : a rudimentary 

 lamina in front of and outside the first laminae, and an extra cusp joined in between 

 the first and second laminae. 



