AMPHIBIOUS MAMMALS 443 



These are nearly all placental mammals — there are but one aquatic mon- 

 otreme (Ornithorhynchus) and one aquatic marsupial, the water opos- 

 sum iChironectes). 



Of those which are amphibious — that is, excluding the whales and sea- 

 cows — the vast majority on Howell's list are small members of the In- 

 sectivora and Rodentia. Howell believes that these, and the platypus, 

 probably keep their eyes closed when submerged. The white bear and 

 several mustelid carnivores are piscivorous and must use their eyes under 

 water, but only for the European otter (Lutra vulgaris) have the eyes 

 been described in any detail. The only other amphibious mammals whose 

 eyes have had any great attention are the beaver, the hippopotamus, and 

 particularly the Pinnipedia — seals, sea-lions, and walruses. We will con- 

 sider these types in the order of their perfection of visual air-and-water 

 adaptation : 



The hippopotamus stands at the bottom of the list. From the situation 

 in the frog and the alligator, we should at once become suspicious upon 

 noting that the hippo's eyes are elevated like his nostrils, and may thus 

 be kept in the air while the rest of the body is submerged. This is an 

 aquatic adapation right enough, but it is definitely not an adaptation to 

 aquatic vision. The adnexa have become modified, though not nearly as 

 much as in the sea-cows. The lids form a ring bearing only traces of 

 lashes, and close as the eye retracts. The nictitans is reduced and can 

 cover only half of the cornea. The nasolacrimal duct has disappeared. 

 No known aquatic or amphibious modifications occur in the eyeball it- 

 self. The hippo is unique in that it spends most of its time in the water, 

 but does all of its feeding on land. In keeping with these habits, the pupil 

 is horizontal like that of terrestrial ungulates, to extend the visual field 

 somewhat in the plane in which the animal needs most of its wariness. 

 The raising of the orbits clearly represents a device for keeping the eyes 

 in their only appropriate medium for as much of the time as possible. 



Studies on the eye of the Canadian beaver are not yet complete. The 

 animal is essentially diurnal when quite unmolested, but does not have 

 the yellow lens (see p. 204) characteristic of other diurnal rodents. 

 Though the eyes are stated by Howell to be the most dorsally-directed 

 among all rodents, the orbits form no turrets as in the hippopotamus. 

 There is thus no a priori reason to suppose that the beaver cannot see in 

 the reduced light under water. The European beaver is stated to have a 

 cornea much thicker than the sclera, which perhaps helps the eyeball to 

 withstand the impact on the water surface, in diving. The ciliary body 



