MUS 633 
together. The median tubercles, x, y, and z are very largely 
developed. In mm’ the lamin are conspicuously bowed, cusp 2” 
lying distinctly behind 4 and in line with cusp y, cusp 6 being 
similarly in line with z; cusps 1, 4, and 5 are large, and clearly 
separated from the median tubercles ; and there is no trace of 
either cusps 3 or 7. In wm’ cusps 4 and 5 are more reduced 
than in wz’, being more intimately connected with cusps y and 2 
than in the latter tooth; cusp 1 is represented solely by a 
minute vestige at the base of the crown, or it may be wholly 
lacking ; cusps #’ and 6 are largely developed, the latter being 
in line with z asin m'. The last molar above is very small, 
and consists of a large cusp 2’ and a postero-external tubercle, 
which appears to be a compound of cusp y and other elements. 
In some species 7° 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 2) is 
obliterated. In m, cusp 2 is relatively small, and there is no 
trace of an anterior median “accessory ” tubercle ; the posterior 
“accessory” (cusp 1) is well developed in this tooth and in 
M; m, consists of three cusps, viz., 2 and 5 in front, and a 
tubercle compounded of y 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 m=: have become of less 
functional importance; they are therefore reduced in size, 
and mw 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. Il. 258 
