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122 BULLETIN :U, UNITKD STATKS NATIONAL MUSEUM. 



from the arj,nimeiit. Hero it can be safely asseitetl that, as far as tlie 

 skeletons are concerned, there exist no other generic distinctions than 

 those given above. If, now, any -ciples can be derived from consid- 

 eration of the osseons system, th. ch of all others ])res(Mits ns with 



by far the greatest number of u\ .0 modifications of structure, the 



same may be with considerable probability inferred for the other sys- 

 tems. 



The primary groups are distinguished by the different degrees of at- 

 tachnuMit of the tongue. That form which is most attached rej)reseuts 

 and is i(h>,ntical with an imnmture stage of the species of section second, 

 where it is more extensively free, as any oneniay satisfy himself by the 

 examination of a larva of Spelerpes at a certain period. The tongue 

 will be found to be tlmt of Tlethodon. 



The secondary groups are distinguished by the separation or cou- 

 Huence of the premaxillary bones. Those presenting the latter type 

 exhibit separate premaxillaries at the beginning of larval life, though 

 the union often takes place very early. The mimber of digits dis- 

 tinguishes groups of genera of less value; in some tiie hind limb has 

 five digits, in others four. In an early larval stage all possess l>ul four 

 digits, and in some of those with live the inner consists of one i)hal;nige 

 only even at maturity {Spdcrpes chiropfcrK.s ct. aff.) Not lli'.^ iug as c(mii- 

 plete a series of larvae of Spelerpes and IMethodon as uf Anihli/stDiiKi 

 imndatum, I describe the develoi)ment of the digits in the latter as iudi- 

 eating the meaning of variations in the same at maturity. At a leugth 

 of l.L"""' the fore limb only is projected, ami bears two digits only, as in 

 the genus Proteus. At 1.5""" sometinH\s the posterior limbs are devel- 

 oped, sometimes imt, and from this size to U.-'i"'"' the nund)er of digits 

 bears little relation to the size of the animal, an additional digit sonu'- 

 times appearing earlier, sometimes later. Their numl)ers aie then at 

 first 2-0; then always 3-0. With the hind foot divided, they ar(^ .'{-L». 

 and then ,'5-1. Sometimes the anterior digits are couiplete iu number 

 before the hind limb api)ears, and we liave coml)iuations of numbers 

 from 1-0 to 1-3, 4-t, and the full nund)er, 1-5, which is found in all 

 specimens of 2.5"'"' and upwards, denera which exhibit reduced digits 

 are in all other respects Spelerpes (/. e. ^Manculus) or Plethodon (lleiui- 

 dactylum), or riemidactylinm with uuossified parietal bones and eonsdli- 

 dated premaxillaries (Hatrachoseps). Api)lying the case of Anibly- 

 stoina to tiiese, we could not assert that Ilemidactylium, for instance, 

 is identical with the undeveloped stage of Plethodon, since when Ambly- 

 stonui exhibits digits l-t it is branch iferous. IJi.t making the more 

 legitimate eomi)arison with Plethodon itself, I find that the complete 

 number of i)osterior digits appears much later in life than in Ambly 

 stoma, while thebranchia' are absorbed much earlier; that development 

 in the first regard is retarded, while in respect to the gills it is a(!cel- 

 erated. Thus in riefhodon ciin'rcus the exterior digit is longer than 

 the interior; iu si)ecimensof 2.1""" the outer di.":it is the shorter; in those 



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