566 MARSUPIALS xx. 4- 



similarities to Thylacinus. *Thylacosmilus was a Miocene sabre-tooth, 

 the size of a panther, whose huge upper canine and other features closely 

 parallel the placental *Smilodon (p. 689). It is said that in some of these 

 borhyaenoids, two or more milk teeth were replaced ; if true this sug- 

 gests that the condition in modern marsupials is secondary. 



5. Marsupial ant-eaters and other types 



Myrmecobius is an ant-eating form, with elongated snout (Fig. 344). 

 Notoryctes, the marsupial mole (Fig. 345), from South Australia, has 

 reduced eyes, well-developed fore-limb, fused cervical vertebrae, and 

 many features suiting it for burrowing and feeding upon ants. Its 

 pouch opens backwards. Sminthopsis, the pouched mice (Fig. 346), 

 are small marsupials occupying the niche taken in other parts of the 

 world by the shrews. 



Caenolestes y the opossum-rat of the forests of the Andes, is an 

 interesting shrew-like creature, with the polyprotodont number of 

 incisors but procumbent lower incisors, resembling those of diproto- 

 donts. There is no syndactyly. It is the survivor of a group formerly 

 abundant in South America, some with teeth similar to those of 

 multituberculates. 



6. Phalangers, wallabies, and kangaroos 



The diprotodont marsupials form a compact group (leaving out 

 Caenolestes) of Australian forms, here included as the superfamily 

 Phalangeroidea. Their fossil history is little known, but they have 

 become specialized for various modes of life, mainly as herbivores, in 

 Australia and the neighbouring islands. The kangaroos and wallabies 

 (Macropodidae) (Fig. 347) have become mostly terrestrial and 

 developed a bipedal method of progression, involving modification of 

 the ilia and thigh muscles, for whose attachment the tibia bears a 

 marked anterior crest. The foot gains increased leverage by elongation 

 of the metatarsal of digit 4. Digits 2 and 3 are very small and syn- 

 dactylous. There are several modifications for a herbivorous diet; the 

 single pair of lower incisors is directed forwards and their sharp inner 

 edges can be moved in such a way as to cut grass like shears. This con- 

 dition recalls that of Rodentia (p. 655) and, as in that group, a special 

 transverse muscle is developed (m. orbicularis oris), but in this case it 

 is part of the facial musculature and innervated from the seventh nerve, 

 whereas the analogous muscle of the rodents is a part of the mylohyoid 

 and innervated from the trigeminal. The molar teeth are modified 

 for grinding, by the fusion of the cusps to make two transverse ridges, 



