122 BULLETIN 34, UNITED STATES NATIONAL MUSEUM. 



from the argument. Here it cau be safely asserted that, as far as the 

 skeletons are concerned, there exist no other generic distinctions than 

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

 eration of the osseous system, that which of all others presents us with 

 by far the greatest number of minute 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- 

 tachment of the tongue. That form which is most attached represents 

 and is identical with an immature stage of the species of section second, 

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

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

 will be found to be that of Plethodon. 



The secondary groups are distinguished by the separation or con- 

 fluence 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 number of digits dis- 

 tinguishes groups of genera of less valuej in some the hind limb has 

 five digits, in others four. In an early larval stage all possess but four 

 digits, and in some of those with five the inner consists of one phalange 

 only even at maturity {Spelerpes chiropteruset. aff.) ISot having as com- 

 plete a series of larvae of Spelerpes and Plethodon as of AmUystoma 

 ])U7iGtatum, I describe the development of the digits in the latter as indi- 

 cating the meaning of variations in the same at maturity. At a length 

 of 1.2'""^ the fore limb only is projected, and bears two digits only, as in 

 the genus Proteus. At l.S™'" sometimes the posterior limbs are devel- 

 oped, sometimes not, and from this size to 2.5°"" the number of digits 

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

 times appearing earlier, sometimes later. Their numbers are then at 

 first 2-0; then always 3-0. With the hind foot divided, they are 3-2, 

 and then 3-4. Sometimes the anterior digits are complete in number 

 before the hind limb appears, and we have combinations of numbers 

 from 4-0 to 4-3, 4-4, and the full number, 4-5, which is found in all 

 specimens of 2.5™"^ and upwards. Genera which exhibit reduced digits 

 are in all other respects Spelerpes {i. e. Manculus) or Plethodon (Hemi- 

 dactylum), or Hemidactylium with unossified parietal bones and consoli- 

 dated premaxillaries (Batrachoseps). Applying the case of Ambly- 

 stoma to these, we could not assert that Hemidactyhum, for instance, 

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

 stoma exhibits digits 4-4 it is branchiferous. But making the more 

 legitimate comparison with Plethodon itself, I find that the complete 

 number of posterior digits aj^pears much later in life than in Ambly- 

 stoma, while the branchi.T are absorbed much earlier; that development 

 in the first regard is retarded, while in respect to the gills it is accel- 

 erated. Thus in Plethodon cinereus the exterior digit is longer than 

 the interior; in specimens of 2.4™'" the outer digit is the shorter; in those 



