NATURAL SCIENCES OF PHILADELPHIA. 95 



Section II. The tongue free all around, attached by its central pedicel only. 

 (Spelerpes). 



A. Two premaxillary bones (with fontanelle). 



Digits 4 — 5, closely united by a broad palmar membrane Geotbiton. 



Digits 4 — 5, entirely free Gtkikophilus. 



AA. One premaxillary bone (with fontanelle). 

 a. Digits 4 — 4. 



Digits free, parietal and palatine bones well ossified Manculus. 



eta. Digits 4 — 5. 



Digits little distinct j parietal cartilages not ossified ; two patches on para- 

 spbenoid teeth , Opheobatrachus. 



Digits entirely confounded as an undivided palm or sole ; cranial bones 

 well ossified CEdipos. 



Digits all free, cranial bones well ossified Spelerpes. 



The generic relationships of the above named groups are exceedingly sim- 

 ple, and the ease with which the animals can be analysed, renders the case clear 

 from the doubts which constantly arise in discussions of generic relationships 

 as to the probable omission of characters from the argument. Here it can 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 consideration of the osseous system, that which of all others 

 presents us with by far the greatest number of minute modifications of struc- 

 ture, the same may be with considerable probability inferred for the other 

 systems. 



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

 ment of the tongue. That form which is most attached, represents, and is 

 identical with, an immature stage of the species of Sect, 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 confluence of 

 the premaxillary bones. Those presenting the latter type exhibit separate 

 premaxillaries during the early portion of larval life, though the union often 

 takes place very early. The number of digits distinguishes groups of genera 

 of less value ; in some the hind limb has five digits, in ethers 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 c h i r o p- 

 ter u s, et aff.), not having as complete a series of larvae of Spelerpes and 

 Plethodon, as of Amblystomap u n c t a t u m,the development of the digits in the 

 latter will indicate the meaning of variations in the same at maturity. At a 

 length of 1-2 mm. the fore limb only is projected, and bears two digits only, 

 as in the genus Proteus. At 1-5 mm. sometimes the posterior limbs are de- 

 veloped, sometimes not; and from this size to 2-5 mm. the number of digits 

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

 appearing earlier, sometimes later. Their numbers are then at first 2 — ; then 

 always 3 — ; with the hind foot divided, they are 3 — 2, and then 3 — 3. Some- 

 times the anterior digits are complete in number before the hind limb appears, 

 and we have combinations of numbers from 4 — to 4 — 3, 4 — 4, and the full 

 number 4 — 5, which is found in all specimens of 2.5 mm. and upwards. 

 Genera which exhibit reduced digits are in all other respects Spelerpes (i. e. 

 Manculus) or Plethodon (Hemidactylium) or Hemidactylium with unossified 

 parietal bones and consolidated premaxillaries (Batrachoseps). Applying the 

 case of Amblystoma to these, we could not assert that Hemidactylium, for in- 

 stance, is identical with the undeveloped stags of Plethodon, since when Am- 

 blystoma exhibits digits 4 — 4, it is branchiferous. But making the more 



1869.] 



