682 Comparative Morphology ol' Ghordates 



(e.g., the cat's) to "shine in the dark." In ungulates the reflecting 

 elements of the tapetum are microscopically delicate fibers arranged 

 compactly in concentric order — the tapetum fibrosum. The carni- 

 vore tapetum consists of several layers of flattened cells, each cell 

 nearly filled with minute rods which appear to be crystalline, although 

 the nature of their substance is not known. This tapetum cellulosum 

 is highly developed in those nocturnal primates, the lemurs. Either 

 type of tapetum occurs in various mammals other than ungulates and 

 carnivores, but less commonly. 



The pupil is usually approximately circular but may be elliptic 

 with its long axis either horizontal or vertical. When, in strong light, 

 the elliptic pupil narrows, it approaches the form of a slit. For protec- 

 tion of the eye against entrance of strong sunlight, an elliptic pupil, 

 capable of narrowing to a slit, is more effective than a circular pupil 

 whose sphincter muscle is incapable of reducing the pupillary diameter 

 to zero. Horizontally elongated pupils occur, for the most part, in 

 herbivores, especially grazing animals, and in other animals whose 

 mode of life makes it important for them to have a wide horizontal 

 range of vision — marsupial kangaroos and wallabies, many ungulates, 

 whales. The domestic cat, with a keen eye for birds, has frequent 

 occasion to look aloft; a good up-and-down sweep of vision is useful. 

 Vertically elongated pupils occur in the Australian "marsupial 

 cats" (dasyures), in common cats and many of the smaller carnivores, 

 in seals, and also in lemurs. 



Whether or not an animal possesses the ability to discriminate 

 colors — i.e., has "color vision" — is always difficult to prove. So far 

 as present evidence goes, it would appear that, regarding the mam- 

 malian Class as a whole, color vision is developed to no higher level 

 than in reptiles. It is only among primates that any wide range of 

 color vision is certainly known to exist. Among reptiles, both the 

 lizards and chelonians have some degree of color vision. Mammals 

 seem to stand far below birds in this respect, for there is very satis- 

 factory evidence that birds have good color vision. But it is possible 

 that color vision may exist in many wild mammals whose eyes have 

 not as yet been studied. 



In capacity for accommodation of the eye to seeing objects at 

 varying distances, most mammals seem to be, at best, not superior to 

 reptiles and decidedly inferior to birds. So far as is known, highly 

 efficient accommodation exists only in primates. The eye of the domes- 

 tic cat accommodates fairly well, and it is probable that some degree of 

 accommodation occurs in other carnivores, especially the larger and 

 the aquatic, and possibly in some ungulates. There is evidence that 



