116 



BMND VERTEBRATES AND THEIR EYES. 



CHOLOCASTER ACASSIZII. 



Only a single specimen of this species appears to have been put on record. 

 Putnam described it from Lebanon, Tenn. The present account is based on five 

 specimens secured by me in the river Styx in Mammoth Cave and in Cedar 

 Sinks. 



The eye of Cholo^islcr agassizii Putnam is much smaller than that of C. 

 ptipillifcnts. In a specimen 41 mm. long it is placed 2.08 mm. from tip of the 

 snout, the eye measuring 0.72 mm. in diameter. The distance from eye to eye 

 is 2.72 mm. It is elliptical in outline, with the lateral face depressed. It is directed 

 outward. The optic nerve, which, at its origin, is surrounded by pigment for a 

 distance of 2.4111111., extends almost straight inward. The dermis over the eye is 

 essentially as in pupillifcnis. The epidermis is less simplified. It is thinner than 

 in the surrounding tissue, but goblet cells are found in it, although they are much 



I 1 ii.. sS. ( ~/;/>/i'i:./i/,T ago 



(n) Cross Mtii, m ol Part of Head. 



IM Vim, ..I Sri!i (t n through Retina of a Specimen 38 mm. long. 



ir ) Vertical Set linn through Retina of :t spr, mi. n t,2 mm. long. Rods, Cones, and Pigment Layers omitted. 



(it) Lower part of Iris of the Same Specimen, 02 mm. long. 



i Her and much less numerous than elsewhere. The sclera and choroid are as 

 in fHipillifiriis, including a pigment mass below the exit of the optic nerve just 

 within (lie sclera. The optic nerve measures 24 fj. at its point of entrance into the 

 pigment layer of the retina, and it is thus one-fourth smaller in diameter than in 

 papilliferus. 



The proportionate thickness of the retinal layers as compared with the 

 layers of /;<//> illifrnis is seen in the table. The maximum thickness in the largest 

 specimen is but rp /i as compared with 166 p in /><i/)illifcnis. This differ- 

 ence is almost entirely due to the thickness of the pigment layer, which is 74 p 

 in the largest agassizii and 104 ^ in the largest f>ii/>il/ifcnts, leaving a difference 

 o| IMII (i ft in the other layers. The pigmented layer is, on an average, much thinner 



