1881.] Himalayas, Tibet, and Afghanistan, 91 



characters of the molar teeth, and especially on the* number of prisms, 

 triangles, or spaces on the crowns, and the number of external and internal 

 salient angles on the sides of each tooth. In estimating the number of 

 prisms or spaces (Schmelzschlingen), those on different sides of the tooth 

 are counted separately, unless they are exactly opposite to each other ; 

 whilst in estimating the salient angles, all that form a distinct fold on the 

 outside of the tooth are enumerated, although, when the teeth are but 

 little worn, such angles are not conspicuous on the crown. As in most 

 other systems of biological definition, it is impossible to obtain absolute 

 uniformity, since it is often a question whether the spaces on the crown of 

 the tooth enclosed by angles on opposite sides correspond or alternate, and, 

 consequently, whether they are to be counted as two or as one. An in- 

 stance is afforded by the lower molar teeth of A. sikimensis (see Plate I. 

 fig. A). In the first tooth it is difficult to say whether the number of 

 spaces or prisms should be counted as 7, 8, or 9, the latter number, how- 

 ever, would best agree with the general practice ; but in the hindmost lower 

 molar we might by this plan count 5 prisms, whereas, in accordance with 

 the usual plan, the number is but 3. To avoid this difficulty some writers 

 enter into details on the construction of the separate teeth*, but this 

 makes the description rather long. 



In counting the external and internal angles also, it is difficult at 

 times to say whether a mere convexity, that does not form a distinct 

 angle, or a comparatively rudimentary fold, such as is frequently found in 

 the anterior portion of the first lower molar, should be included or not ; 

 and there are sometimes individual variations within the limits of the same 

 species. Good figures are the only safeguards against misunderstanding. 



The European forms are thus divided by Blasius : — 



I. Molars furnished with roots in the adult animal. 



A. HxPUDiEUS, Illig. 



First lower molar with 7 spaces, 4 outer and 5 inner angles ; second lower 

 molar with 3 divided spaces and 3 internal and external angles ; second 

 upper molar with 4 spaces, 3 outer and 2 inner angles. The inter- 

 parietal bone throughout the whole breadth of its hinder margin 

 raised in a flat convexity, a pointed projection in the middle of the 

 bone in front, and a long gradually diminishing point on each side. 

 The European forms are A. glareolus and A. rutilus. 



II. Molars rootless. 



B. Paludicola. 



First lower molar with 7 spaces, 4 outer and 5 inner angles ; second lower 

 molar with 5 simple spaces and 3 internal and external angles ; second 



* For instance, Coues and Allen, ' Monograph of North- American Iiodentia,' 

 Muridco, genus Arvicola. 



