The Sirenidae are the most degraded members of the Urodela and are represented by two closely-allied genera, each with one species, in the south-eastern parts of the United States. Their most interesting feature, which bears upon the question of neoteny, is their retrograde metamorphosis as described by Cope.[67] The gills atrophy in the young and are subsequently redeveloped. Cope therefrom concludes rightly that the ultimate or persistent gills of _Siren_ are signs of maturity and not a larval character. In young specimens of _Siren_ of 5 to 6 inches in length the gills are functionless; in one of 3 inches they were found to be entirely vestigial and "subepidermal," _i.e._ covered by a common dermal investment. Unfortunately really young larvae are still unknown. Old Sirens can live without gills, as has been shown by aquarium-specimens. In the adult _Pseudobranchus_ all the gills are normally covered up by an investment of the skin so as to be quite without function and movability. _Siren lacertina_, the "mud-eel," is distinguished by the {137}possession of three pairs of gill-clefts and by its four fingers. It reaches a length of 70 cm., or about 2½ feet, of which about one-third is taken up by the tail, which is strongly compressed and finned. The skin is smooth, mostly blackish, lighter below, sometimes with whitish specks all over the body. This creature is frequently found in ditches and ponds, where it burrows in the mud. When swimming the limbs are folded back. They are said sometimes to leave the water and to crawl about on the moist ground. _Pseudobranchus striatus_ has only one pair of gill-clefts and only three fingers. The slightly granular skin is dusky brown above, with a broad yellow band on either side and with a paler, narrower stripe below. Total length about 7 inches. {138}CHAPTER VI LISSAMPHIBIA (_CONTINUED_)–ANURA ORDER III. ANURA OR TAILLESS AMPHIBIA. The recent tailless Amphibia, or Frogs and Toads in the widest sense, contain such a great number of species (about 900), with such a diversity of characters, that it is necessary, if only for the sake of mere convenience, to group them into a considerable number of families and sub-families. The characters available for this purpose are few. 1. The possession of a tongue characterises the PHANEROGLOSSA, the absence of a tongue the AGLOSSA. 2. The character of the shoulder-girdle.–Overlapping of the two halves of the shoulder-girdle on the ventral side characterises the ARCIFERA, while in the FIRMISTERNIA the two ventral halves meet in the middle line and form a firm, median bar. See, for details, p. 24. 3. The shape of the transverse processes or diapophyses of the sacral vertebra which carries the iliac or hip-bones. These processes are either _dilated_ or _cylindrical_.