194 VERTEBRATE ZOOLOGY 



Typhlomolge rathbuni (Fig. 113, C) a form very much like Proteus, is 

 a native of Texas. It inhabits subterranean eaves and is sometimes 

 brought to the surface in water from artesian wells. It is likely that 

 both of these cave mud-puppies arose independently, in response to 

 similar conditions, from some form like Necturus. 



Family 4. Sirenidoe. (The Sirens.) — They have three pairs of per- 

 manent fringed external gills; the body is eel-like, and there are no 

 hind limbs; maxillaries are absent; no teeth are present, except some 

 small ones on the vomer; jaws are furnished with a horny sheath like 

 that of frog larvae; there are no eye-lids. There are two genera, each 

 represented by a single species. 



Siren lacertina (Fig. 113, D),the "mud-eel," has three pairs of gill- 

 clefts and external gills in the adult. It reaches a length of two and 

 a half feet. The tail is strongly compressed and with well-developed 

 fins: The color is blackish above and lighter below. The animal lives 

 in the mud at the bottom of ponds. It is found in the southeastern 

 parts of the United States. 



Pseudobranchus striatus is very much like Siren but is smaller, sel- 

 dom exceeding seven inches in length. It has only one pair of gill- 

 clefts and only three fingers. A broad yellow band along the side re- 

 lieves the somber coloration. 



The Sirenidse are considered the most degraded of urodeles. That 

 they are not truly primitive is borne out by the observation of Cope 

 that the young lose the external gills which then redevelop in the 

 adult; so the adult condition is a renewed or secondary larval con- 

 dition. This and other observations tend to confirm the theory of 

 psedogenesis as applied to both Serenidse and.Proteidae. 



Which of the Anura are to be considered the most primitive, the 

 nearest approach to the first ancestral Amphibia? It seems certain 

 that the perennibranchiate forms are not ancestral but merely re- 

 tain, or return to, a larval condition. Probably Cryptobranchus rep- 

 resents a condition more nearly primitive or ancestral than any 

 other living amphibian. 



PjEDOgbnesis or Neoteny 



There is perhaps no better group of vertebrates for illustrating the 

 phenomenon of pedogenesis or the retention of larval structures 

 during sexual maturity. It is a common phenomenon in a number 

 of groups of invertebrates and especially so among the most highly 



