406 



THE NATURAL HISTOET llEYIEW. 



The third milk molar (Dm 3) is only to be differentiated from the 

 fourth by its smaller size (Figs. 3 and 5). In both the external 

 lamina, L, bears two costse on its anterior area, of which the second is 

 the higher. The groove between them is deep and vertical. The 

 posterior area, N, is faintly undulating and bears but the faintest 

 trace of the costse visible in the corresponding teeth of B. ticTiorhU 

 nus. The entrance of the anterior valley, A, is wide, and the comb- 

 ing plates, G and H, sometimes map off an accessory valley C. In 

 some cases the head of the valley is more or less filled with accessory 

 folds of enamel. The posterior valley is small. The inner side of 

 the anterior and median colles, D and E, slopes abruptly from the 

 base towards the summit of the crown. The guard or obliquely 

 ascending ridge of enamel on the anterior aspect of the tooth is very 

 strongly developed, and circumscribes a deep pit at the inner angle 

 of the anterior coUis. On the base of the external lamina of the 

 fourth milk molar (Fig. 4) is a small abnormal cusp. 



§ 3 B. Lower Milk Molars. — The large size and the slight deve- 

 lopment of the ribs on the anterior area differentiate the three last 

 milk teeth from the tichorhine homologues, the former character 

 from the leptorhine. 



The first small trenchant milk molar (Fig. 6.) 

 presents but the faintest shadow of the structure 

 obtaining in the rest of the lower milk series. 

 The external surface or lamina, L, is tumid, with 

 a broad ill defined median ridge bounded on 

 either side by a faint depression as in its ticho- 

 rhine homologue. Of the anterior valley, A, 

 there is but the merest trace, and the posterior 

 is but slightly mapped off. The median collis, 

 E, is small and very oblique, the anterior can 

 hardly be said to exist at all. The two fangs 

 are stout and cylindrical, and show no trace of 

 the pressure of a successional tooth. 

 In the second milk molar (Eigs. 7, 8), the external lamina, L, is 

 divided into two arese, M and N, by the median groove of which the 

 larger, the posterior, is tumid, and projects more outwards than the 

 anterior. The latter bears two ill-defined costa\ The anterior valley, 

 A, is more shallow than the posterior, and has its entrance at a much 

 lower level. On its posterior wall is a slight fold. The anterior 



