=Amblyopsidæ.=—The cavefishes, _Amblyopsidæ_, are the most remarkable of the haplomous fishes. In this family the vent is placed at the throat. The form is that of the _Pœciliidæ_, but the mouth is larger and not protractile. The species are viviparous, the young being born at about the length of a quarter of an inch. [Illustration: FIG. 161.—Dismal Swamp Fish, _Chologaster cornutus_ Agassiz. Supposed ancestor of _Typhlichthys_. Virginia. ] In the primitive genus _Chologaster_, the fish of the Dismal Swamp, the eyes are small but normally developed. _Chologaster cornutus_ abounds in the black waters of the Dismal Swamp of Virginia, thence southward through swamps and rice-fields to Okefinokee Swamp in northern Florida. It is a small fish, less than two inches long, striped with black, and with the habit of a top-minnow. Other species of _Chologaster_, possessing eyes and color, but provided also with tactile papillæ, are found in cave springs in Tennessee and southern Illinois. [Illustration: FIG. 162.—Blind Cave-fish, _Typhlichthys subterraneus_ Girard. Mammoth Cave, Kentucky. ] From _Chologaster_ is directly descended the small blindfish _Typhlichthys subterraneus_ of the caves of the Subcarboniferous limestone rocks of southern Indiana and southward to northern Alabama. As in _Chologaster_, the ventral fins are wanting. The eyes, present in the young, become defective and useless in the adult, when they are almost hidden by other tissues. The different parts of the eye are all more or less incomplete, being without function. The structure of the eye has been described in much detail in several papers by Dr. Carl H. Eigenmann. As to the cause of the loss of eyesight two chief theories exist—the Lamarckian theory of the inheritance in the species of the results of disuse in the individual and the Weissmannian doctrine that the loss of sight is a result of panmixia or cessation of selection. This may be extended to cover reversal of selection, as in the depths of the great caves the fish without eyes would be at some slight advantage. Dr. Eigenmann inclines to the Lamarckian doctrine, but the evidence brought forward fails to convince the present writer that results of individual use or disuse ever become hereditary or that they are ever incorporated in the characters of a species. In the caves of southern Missouri is an independent case of similar degradation. _Troglichthys rosæ_, the blindfish of this region, has the eye in a different phase of degeneration. It is thought to be separately descended from some other species of _Chologaster_. Of this species Mr. Garman and Mr. Eigenmann have given detailed accounts from somewhat different points of view.