253 
An adult and a larva taken from Marble Cave were sectioned in the 
usual manner. The lens and iris in both were normal. The only differ- 
ence in the histological structure of the eye, when compared with the 
normal salamander (Amblystoma jeffersonianum), is found in the retina. 
In the larvae all the layers of the retina are well developed. The 
ganglionic layer is much thicker than that of the Amblystoma, having 
many rows of cells instead of one or two. All the other layers are nor- 
mally present, the rod and cone layer being well developed. The retina 
in the larva is much thicker than in the adult. In the adult the rods 
and cones have disappeared, there being only an occasional process from 
the outer nuclei. 
In all the sections thus far studied we have been unable to detect 
the slightest indication of an outer molecular layer in the adult, while 
in the larva this layer is normally developed. The ganglionic layer is 
thicker in the larva than in the adult. In this respect the adult ap- 
proaches the normal more than the larva does. The Miillerian fibres are 
profusely present in both larva and adult. 
SUMMARY. 
1. The larval retina approaches the normal (Amblystoma) more than 
the adult.” The only apparent difference is a thickening of the ganglionic 
layer. 
2. The retina is thicker in larva than in adult. 
3. All the layers are present in the retina of the larva, while in the 
adult the rods and cones and the outer molecular layer have not been 
made out; the inner molecular layer is thinner. 
4, The ganglionic layer is thicker in larvae than in adult. 
THe Bruinp Rat or Mammotu Cave.* By Cart H. EIGENMANN AND 
JAMES ROLLIN SLONAKER. 
Hapits AND Hapirat, By Cart H. ErGenmann. NO. 22. 
In his origin of species, sixth edition, Vol. I, page 171, Darwin says 
that the eyes of Neotoma of Mammoth Cave are “lustrous and of large 
size; and these animals, as I am informed by .Prof. Silliman, after having 
been exposed for about a month to a graduated light, acquired a dim per- 
*Qontribution from the Zodlogical Laboratory of the Indiana University. 
