THE BLIND CAT-FISH. 69 



in the fact that the choroid gland is composed entirely of pigment ; in the fact that the iris, though 

 of fully the normal thickness, is almost entirely of pigment, there being on its outer surface in some 

 specimens a small amount of cellular material, which probably represents the ligamentum annulare; 

 in the great proportional thickness of the pigment layer of the retina and the entire absence in it of 

 anything excepting pigment; in the incomplete differentiation of the layers of the retina, there being 

 in some individuals scarcely more than a trace of the external reticular layer separating the two 

 nuclear layers, and there being in no specimen studied a retina sufficiently developed to enable one 

 to homologize with certainty the layers marked out ; in the minute size of the optic nerve, and the 

 fact that it is ensheathed in a thick layer of pigment for nearly its entire course through the retina; 

 and, finally, in the small size of the motores oculi. 



6. The surest evidences of actual degeneration are found, first, in the greatly augmented quantity 

 of pigment in all the parts that are at all pigmented in the normal eye ; and, secondly, in the presence 

 of pigment in regions where none is found in the normal eye, as in the hyaloid membrane. 



No undoubted instances of degeneration through the breaking down and dissolution of the 

 tissue without the formation of pigment, such as have been described particularly by Looss, have 

 been found, though in a single specimen (the one in which no lens is present) a process of this nature 

 may be taking place. 



THE EYES OF THE BLIND CAT-FISH, AMEIURUS NIGRILABRIS. 



All that is known of this fish is contained in the following extract from Cope's 

 paper (Proc. Acad. Nat. Sci., Phila., 1864, p. 231): 



For a knowledge of the first genus of blind Silurid from our country, I am indebted to my friend 

 Jacob Stauffer, secretary of the Linnamn Society of Lancaster, an ardent explorer of the zoology 

 and botany of southern Pennsylvania, and who has furnished me with many valuable notes and 

 specimens. This fish, of which specimens have been taken in the Conestoga Creek, a tributary of the 

 Susquehanna, is simply a blind representative of the ordinary type of Silurids, characteristic of 

 North America, and is not to be arranged with the exotic groups. * * * The color of the upper 

 surfaces, tail, fins, barbels, and under jaw is black; sides varied with dirty yellow, abdomen and 

 thorax yellowish white. * * * A specimen died in 20 minutes after capture, when put in water, 

 though uninjured ; the A meiuri, like other cat-fishes, will live for many hours after complete removal 

 from their element. It is occasionally caught by fishermen, and is supposed to issue from a 

 subterranean stream, said to traverse the Silurian limestone in that part of Lancaster County and 

 discharge into the Conestoga. 



Two specimens of this fish present an interesting condition of the rudimental eyes. On the left 

 side of both a small perforation exists in the corium, which is closed by the epidermis, representing 

 a rudimental cornea; on the other the corium is complete. Here the eyeball exists as a very small 

 cartilaginous sphere with thick walls, concealed by the muscles and fibrous tissue, and filled by a 

 minute nucleus of pigment. On the other the sphere is larger and thinner walled, the thinnest 

 portion adherent to the corneal spot above mentioned ; there is a lining of pigment. It is scarcely 

 collapsed in one, in the other so closely as to give a tripodal section. Here we have an interesting 

 transitional condition in one and the same animal, with regard to a peculiarity which has at the same 

 time physiological and systematic significance, and is one of the comparatively few cases where the 

 physiological appropriateness of a generic modification can be demonstrated. It is therefore not 

 subject to the difficulty under which the advocates of natural selection labor, when necessitated to 

 explain a structure as being a step in the advance toward, or in the recession from, any unknown 

 modification needful to the existence of the species. In the present case observation on the species 

 in a state of nature may furnish interesting results. In no specimen has a trace of anything rep- 

 resenting the lens been found. 



