252 THE AMERICAN NATURALIST. [Vor. XXXV. 
From this point onwards development takes place along two 
main lines. In one case the outer cusps of the teeth from 
being tuberculate become laterally compressed and trenchant, 
giving rise to the carnivorous modification, seen typically in 
the specialized Creodonta (Oxyzna, Hyzenodon, Oxyznodon, 
etc.). In the other case the primitive tooth pattern is entirely 
changed; the upper teeth become quadritubercular by the 
addition of a postero-internal cusp (hypocone), while the lower 
teeth also become quadritubercular by the reduction of the 
most anteriorly placed cusp (paraconid), and the intermediate 
cusp of the heel (hypoconulid). In this way a primarily omniv- 
orous modification is instituted, which is seen in its construc- 
tive stages in the Eocene Condylarthra (series Protogonodon, 
Euprotogonia, Phenacodus, and various separate genera). 
From the typically quadrituberculate condition evolution 
proceeds in two directions, both leading to the herbivorous 
modifications of the Ungulata. In one series the cusps of 
both upper and lower teeth become crescentic, producing the 
condition characteristic of the Artiodactyla ; while in the other 
they become lophoid, producing the complex modifications of 
the Perissodactyla. The latter evolution is beautifully illus- 
trated in passing from the Eocene genera (Hyracotherium, 
Systemodon, etc.) to the modern types. 
If, now, we arrange the teeth of the Australian marsupials 
according to the above placental plan, we obtain such a result 
as is represented diagrammatically in Pls. I and II. The primi- 
tive trituberculate, tubercular sectorial type is here represented 
by the teeth of Dasyurus viverrinus (Fig. d). As indicated 
elsewhere in a footnote, this would be more appropriately 
illustrated by the teeth of one of the purely insectivorous forms 
of the Dasyuridae (Sminthopsis, Antechinomys, Phascologale). 
The carnivorous line above mentioned is, in the case of the 
Australian marsupials, entirely confined to the family Dasyu- 
ride, and it culminates in the Tasmanian wolf (Thylacinus cyno- 
cephalus). The teeth of this animal are represented in Pls.I 
and II, Fig. ¢; they show all the essential characters of those 
of Dasyurus, except that in the lower teeth the metaconid is 
absent. The progressive carnivorous modification is, in the 
