MUS 633 



together. The median tubercles, x, y, and z are very largely 



developed. In iw' the laminae are conspicuously bowed, cusp ^ 



lying distinctly behind x and in line with cuspjv, cusp 6 being 



similarly in line with z\ cusps i, 4, and 5 are large, and clearly 



separated from the median tubercles ; and there is no trace of 



either cusps 3 or 7. In w* cusps 4 and 5 are more reduced 



than in ni)-, being more intimately connected with cusps jj/ and z 



than in the latter tooth ; cusp i is represented solely by a 



minute vestige at the base of the crown, or it may be wholly 



lacking ; cusps x! and 6 are largely developed, the latter being 



in line with z as in m)-. The last molar above is very small, 



and consists of a large cusp od and a postero-external tubercle, 



which appears to be a compound of cusp y and other elements. 



In some species n^ consists of one tubercle only. In the lower 



molars the median tubercles y and z are so greatly enlarged 



that all trace of the primitive outer row of cusps (6, 7, and n) is 



obliterated. In mi cusp z' is relatively small, and there is no 



trace of an anterior median " accessory " tubercle ; the posterior 



"accessory" (cusp i) is well developed in this tooth and in 



m^; m^ consists of three cusps, viz., z and 5 in front, and a 



tubercle compounded ofy and 4 behind. 



The relatively large size of m^ in the genus appears to be 

 a consequence of the increased strength of the masseter 

 muscles ; the anterior tooth is most favourably placed dynami- 

 cally, and it therefore takes a larger share of the work 

 of mastication. The hinder teeth w|^ have become of less 

 functional importance ; they are therefore reduced in size, 

 and m^ tend to disappear. In South American specimens the 

 latter teeth are frequently lacking, either wholly or from one or 

 the other jaw (see p. 649 below). 



Nothing is definitely known of the geological history of this 

 genus. It has been recorded from the British Pleistocene on 

 several occasions ; whilst most of these records seem to have 

 been based on error, it is just possible that one or two of them 

 may be well-founded. 



VOL. II. 2 s 



