PUPIL SHAPES AND THEIR MEANING 227 



The monotreme mammals are secretive and nocturnal, and have round 

 pupils. Among the marsupials, the kangaroos and wallabies are practi- 

 cally arhythmic, and many have oval (horizontal) pupils. New- world 

 marsupials, and many Australian ones, have round pupils. Other Austra- 

 lian species have the vertical slit. O'Day finds that some marsupial pupils, 

 usually described as round, do finally take on the slit form as the light 

 becomes sufficiently intense. Dasyurus viverrinus shows this well; but the 

 pupil of the more strongly nocturnal Trichosurus vulpecula becomes a 

 small vertical slit even in diffuse daylight of ten to twenty foot-candles, 

 at which intensity the Dasyurus pupil is still circular. 



The placental mammals as a whole are crepuscular and nocturnal, and 

 shun bright light. The hoofed animals and the great cats are arhythmic, 

 while many primates, most squirrels, and a small handful of other scat- 

 tered genera (Ochotona, Zenkerella, Suricata, etc.) are diurnal. Though 

 the squirrels include strongly nocturnal forms (the flying squirrels) as 

 well as sun-worshippers, they all have round or slightly oval (horizontal) 

 pupils. In some of the ungulates — the camel family especially — the cor- 

 pora nigra of the upper and lower pupil margins (Fig. 86) can meet or 

 interdigitate in very bright light, perhaps forming useful stenopaic aper- 

 tures. In others the pupil never approaches a slit form, but can best be 

 described as horizontally rectangular; and it may have only slight mobil- 

 ity, as in the horse. The pupil of a young horse is round, but at five or 

 six years of age it becomes elliptical and the corpora nigra become pro- 

 nounced, three or four of them on the superior border and five or six 

 smaller ones on the inferior border of the pupil. The sheep, with as many 

 as twenty corpora nigra, has the maximum number of these bodies. 



Large carnivores have round pupils. The foxes, all Viverridae except 

 Cynictis and Suricata, and one or two rodents have vertical ellipses. Out- 

 side of the prosimians, a fully closable slit is seen in mammals only in the 

 smaller cats, the strongly nocturnal and arboreal toddy cat or palm civet 

 (Paradoxurus) , and the dormice {Glis spp.). Paradoxurus is excep- 

 tional in having a horizontal slit, which has a single pair of central 

 notches on its margins which form a single stenopaic aperture when 

 the remainder of the pupil closes entirely. Two other viverrids, Cynictis 

 and Suricata, have horizontally oval pupils on the order of those of un- 

 gulates. Suricata, peculiarly vegetarian for a carnivore and rather mar- 

 mot-like in its behavior, is said to be diurnal. 



In the cats and dormice the vertical pupil can also close entirely, leav- 

 ing, in the domestic cat at least, a pair of terminal pinholes reminiscent 



