MAMMALS 



441 



condition which occurs only exceptionally in American Marsupials and 

 it is noteworthy that in American opossums some of the single cones 

 lack oil-droplets. It is also interesting that among Mammals only the 

 Monotremes and Marsupials have either double cones or oil-droplets. 



Fig. 553. — The Visual Cells of an American Marsupial. 



The American mouse-opossum, Marmosa mexicana, showing (from left 

 to right) a single cone with oil-droplet, a single cone without oil-droplet, a double 

 cone with an oil-droplet in one member, and a long filamentous rod ( X 1,000) 

 (Gordon Walls). 



Wombat, 

 Phascolomys 



THE OCULAR ADNEXA have received little attention. A poorly 

 developed nictitating membrane is present, a harderian and a lacrimal 

 gland as well as a retractor bulbi muscle. The Virginian opossum, 

 Didelphys virginiana, has no true nictitating membrane ; two folds of 

 conjunctival tissue arising from either canthus close over the eye in 

 the mid-line while the globe retracts into the socket. In other forms 

 (the bandicoot, Perameles) when the eye is touched the globe rolls 

 backwards and retracts as the nictitating membrane flashes over it, the 

 lids sometimes closing over it at the same time. 



Rabbit bandicoot, 

 Peragale 



THE PLACENTAL EYE 



The PLACENTALS (eutheria) Comprise the vast majority of Mammals 

 and include a multitude of types. These can be arranged in 15 orders, the 

 enumeration of which will facilitate understanding of the subsequent discussion. 



(1) insectivora, the most primitive type of Placentals found widely in 

 temperate and tropical lands except S. America and Australasia (where 

 insectivorous opossums exist). 



The most widely known representatives are the true shrews (Soricidse), the 

 true moles with vestigial eyes (Talpidse) including the water-moles or desmans 

 {Myogale), and the hedgehogs (Erinaceidae). Further types are the otter-shrew 

 of W. Africa {Potamogale), the oriental tree-shrews (Tupaiidse) (sometimes classed 

 among the Prosimians), the elephant-shrews (Macroscelidae) of Africa with very 



