No. i.] BLIND VERTEBRATES OF NORTH AMERICA. 39 



portion of the nucleus is very much elongated. This form is 

 of frequent occurrence. In Fig. 8 is shown the common 

 form where the nuclei are simple elliptical bodies, which give 

 no evidence whatever of any processes uniting them with the 

 other elements of the retina. The Miillerian fibers are pro- 

 fusely present and of very large size in both larva and adult. 

 In both adult and young the optic nerve enters as a single 

 strand and passes entirely through the layers. A heavy mass 

 of pigment is found following the optic nerve to within a short 

 distance of the brain. 



AVERAGE MEASUREMENTS OF THE EYES OF TYPHLOTRITON. 



SUMMARY. 



Typhlotriton is an incipient blind salamander living in the 

 caves of southwestern Missouri. It detects its food by the 

 sense of touch without the use of its eyes. It is stereotropic. 

 Its eyes show the early stages in the steps of degeneration 

 from those of salamanders living in the open to those of the 

 degenerate Typhlomolge from the caves of Texas. The lids 

 are in process of obliteration, the upper overlapping the lower 

 so that the eye is always covered in the adult. The sclera 

 possesses a cartilaginous band in the larval stages but not in 

 the adult. The disappearance of the cartilage is probably an 

 incident of the metamorphosis, not of the degeneration the eye 

 is undergoing. The lens is normal. The retina is normal in 



