62 



THE AMERICAN NATURALIST 



[Vol. XL 



been before the investigation. This retraction movement took 

 place independently of the position of the other bulbus. 



By comhining a number of partial observations, only the fol- 

 lowing- particulars could be made out: a very strongly curved 

 cornea, a considerable mobility of the blackish brown iris which 

 reacted promptly to light, an indistinct radial striation, and a 

 circular pupil. 



The use of the ophthalmoscope seemed at first to be attended 

 with no results, for nothing but a deep black was visible. Then 

 at a chance motion of the dugong's eye the small, round rose- 

 colored papilla or the disc of the optic nerve came in sight. There- 

 upon further exploration of the retina was made though without 

 additional results. The entire area of all four quadrants con- 

 sisted of a deep black continuous tapetum nigrum in which no 

 details could be made out, as is likewise true to some extent in 

 case of the ventral portion of the fundus of the horse. In order 

 to perceive clearly the few extremely delicate capillaries that 

 radiated to the tapetum from the upper rim of the disc required 

 an increase of the observer's myopia from 2} to 5 dioptria from 

 which may be deduced the degree of short-sightedness of the 

 animal when out of water. The refraction of both eyes w^as the 



It is noteworthy that the dugong also exhibited a certain dis- 

 like for bright light. At least one may so conclude from the 

 eagerness with which it sought the shadow cast by the hull of 

 the cutter. 



In the manatee the pupil is round in life, but is transversely 

 oval when the animal is dead (Murie, '80, p. 24). The dugong's 

 pu))il did not change shape after death. As in the manatee the 

 ni( ritatin<,f incmhrane is well developed and freely movable. The 

 evt'lids an- (ontiactile and can be brought together until there is 

 only a small slit, 12 mm. long, that remains not (|uite closed. 

 The complete isolation of the l.nll.ns from the vxtwluv is effected 

 by the nictitating nu'inhraiic an<l the periorbital fat. The lids 



of the very strong orbicular contraction. This fact disposes of 

 Putter's (:03, p. 369) assumption based on the study of an em- 

 bryo dugong whose eye slits were 5 mm. long and 3 mm. broad, 



