MAMMALS 



437 



tarsus in the lower lid only ; the platypus in neither (Newell, 1953). 

 Lacrimal and harderian glands are said to be present in both. The 

 platypus has a well-formed and quite opaque nictitating membrane ; 

 the ant-eater has none. The eye of the echidna, however, has a 

 habit of rolling inwards and retracting into the socket rhythmically, 

 an action aided by squeezing the lids (Johnson, 1901) ; the same pro- 

 tective phenomenon is seen in Edentates, the bandicoot and the 

 porcupine. Both have the usual six extra-ocular muscles in addition 

 to a retractor bulbi muscle ; but the superior oblique muscle is 

 essentially mammalian in type. It will be remembered that in Verte- 

 brates below Mammals the recti take origin from the apex of the orbit, 

 the obliques from its anterior part^; in Monotremes the superior 

 oblique arises close to the origin of the recti and is threaded through a 

 pulley in the supero -medial aspect of the anterior part of the orbit so 

 that it runs sharply backwards towards the temporal aspect of the 

 globe. This typically mammalian form is supplemented in the echidna 

 by a second muscular slip running to the globe directly from the 

 anterior nasal orbital wall, a relic of the sub-mammalian arrangement. 

 The orbit in the platypus is merely a shallow depression at the 

 cephalic extremity of the combined temporo-orbital fossa, provided 

 only with dorsal and median walls and without an interorbital septum 

 — a non-mammalian configuration (Watson, 1916 ; Kesteven and 

 Furst, 1929 ; de Beer and Fell, 1936). There is no optic foramen, for 

 the optic and other cranial nerves leave the skull through a large 

 pseudo-optic foramen (Watson, 1916 ; Hines, 1929). 



THE MARSUPIAL EYE 



The MAESUPiALS (metatheria) — in the Eocene period a large and wide- 

 spread group — are today found only in Australasia with the exception of the 

 American opossums (Didelphyidaj),^ arboreal, rat-like animals found in Central 

 and South America, and the Selvas (Ctenolestes), a primitive family until recently 

 believed extinct, found in South America. In Australasia, however, where 

 competition from the higher carnivorous Mammals has not occurred, there are 

 many forms — (a) the cat-like dasyures (Dasyuridte) (including the squirrel-like 

 banded ant-eater, Myrmecohius, and the Tasmanian devil, Sarcophilus); (b) the 

 burrowing, mole-like Notoryctidsc ^ ; (c) the burrowing, rabbit- or rat-like 

 bandicoots (Peramelidte) ; (d) the squirrel-like arboreal Phalangeridse, including 

 the flying phalangers, Petaurus and Acrobates (Phalangerinse), the bear-like 

 wombats (Phascolomyinfe), and the koala (Phascolarctinje) ; and (e) the unique 

 kangaroos and wallabies (Macropodidse). 



' p. 277, Fig. 293. 



^ Incidentally, among the American opossums, the pouch is generally absent, and 

 the young are carried on the back of the mother, their tails coiled round hers. 



' Notoryctes typhlops, the marsupial mole, has vestigial eyes, less than 1 mm. in 

 diameter, which lack lens, vitreous and visual cells, p. 733. 



Banded ant-eater, 

 Myrmecohius 



Marsupial mole, 

 Notoryctes 



