ø 
71 
through a stronger articulation and a masseter has been developed. 
The anterior teeth still being used for seizing purposes are not 
very modified, whereas. the pøsterior teeth, being placed near 
the muscles, are enlarged. As the upper and lower teeth during 
mastication are rubbed against each other, cusplets are developed 
at the points of contact anterior and posterior to the original single 
conical tooth-cusp. The teeth are now tricuspid (Spalacotherium, 
Amphiterium, Amblotherium, Perolestes, Achyrodon, Triconodon, 
Triacanthodon, Tinodon etc.). These three primary cusps are 
numbered starting from the front cusp 1—2—3; the figures are 
purely topographical, by no means dependent on the relative age of the 
Cusps (the reptilian cone is no. 2.). These tricuspid teeth where the lower 
teeth in biting touch upon the inner side of the upper — the jaws being 
anisognathous — are excellently suited for cutting flesh (the anterior 
premolars of many recent flesh-eaters have still kept this form) ; but the 
mastication of the food demands more than this. Through the friction two 
new Cusps are developed, 4—5, on the inner side of the upper and the 
outer side of the lower tooth according to the accompanying diagram: 
where the Roman numerals indicate the cusps i 1 
of the upper teeth, and the other figures the ma & sp 
Cusps of the corresponding lower teeth. Now I 3 FM ge 3 
the teeth become quinque-cuspid, as is still Ul II 
Seen in the lower jaw of several primitive mammals (Didelphys, 
Dasyurus, Cladobates, Talpa, Chiroptera a. 0.). The lower teeth 
never grow broader, though secondary cusps are sometimes dev- 
eloped at the posterior end. But the upper teeth which are 
Placed in the broad maxilla, acquire, owing to the constant 
necessity for a broad grinding-surface, one cusp more, 6 (just inside 4), 
Senerally supported by a special fang (Didelphys, Talpa a. 0.), and 
Subsequently another, 7, behind 6 (e. g. Sorez). But on this 
broad 7=cuspid tooth the three primary cusps (1—2—3) are pushed 
So far aside that they degenerate and finally disappear, so that 
the tooth becomes quadri-cuspid. On the lower teeth 1 often disappears, 
its function being assumed by cusp 3 of the preceding tooth: 
