The Scyphophori, Haplomi, and Xenomi 195 
extraordinary structure of the eye. This organ is prominent 
and is divided by a horizontal partition into two parts, the 
upper, less convex, adopted for sight in the air, the lower in the 
water. The few species of Anableps are found in tropical 
America. The species of some genera swim near the bottom, 
but always in very shallow waters. Allare very tenacious of life, 
and none have any commercial value although the flesh is good. 
Fig. 152a.—Four-eyed Fish, Anableps dovit Gill. Tehuantepec, Mexico. 
The unique structure of the eye of this curious fish has been 
carefully studied by Mr. M. C. Marsh, pathologist of the U. S. 
Fish Commission, who furnishes the following notes published 
by Evermann & Goldsborough: 
“The eye is crossed by a bar, like the diameter of a circle, 
and parallel with the length of the body. This bar is darker 
than the other external portions of the eyeball and has its edges 
darker still. Dividing the external aspect of the eye equally, 
it has its lower edge on the same level as the back of the fish, 
which is flat and straight from snout to dorsal, or nearly the 
whole length of the fish; so that when the body of the fish is just 
submerged the level of the water reaches to this bar, and the 
lower half of the eye is in water, the upper half in the air. Upon 
dissecting the eyeball from the orbit, it appears nearly round. 
A membranous sheath covers the external part and invests most 
of the ball. It may be peeled off, when the dark bar on the 
external portion of the eye is seen to be upon this membrane, 
which may correspond to the conjunctiva.’ The back portion 
of the eyeball being cut off, one lens is found. The lining of the 
ball consists, in front, of one black layer, evidently choroid. 
Behind there is a retinal layer. The choroid layer turns up 
anteriorly, making a free edge comparable to an iris. The 
free edge is chiefly evident in the lower part of the eye. A large 
pupil is left, but is divided by two flaps, continuations of the 
choroid coat, projecting from either side and overlapping. 
