96 PROCEEDINGS OP THE ACADEMY OP 



legitimate comparison with Plethodoa itself, I find that the complete number 

 of posterior digits appears much later in life than in Amblystoma, that devel- 

 opment in this regard is retarded, while in respect to the gills it is accelerated. 

 Thus in Plethodon e ry th r n t u s the exterior digit is longer than the in- 

 terior ; in specimens of 2-4 mm. the outer digit is the shorter; in those of 1*8 

 mm. which are without gills, it is a very minute tubercle on the outer meta- 

 tarsus. In a little earlier stage it cannot but be wanting, though this I have 

 not seen, and I have little doubt that it is then a Hemidactylium, unless in- 

 deed the parietal bones be not ossified. 



Another foetal condition rendered permanent is seen in the generic charac- 

 ter of the genus ffidipus, which differs from Sptlerpes solely in the foetal non- 

 separation of the digits. In the larva of Sp, rubra, the digits are early 

 entirely distinct, so that so far as this species is concerned Qildipus presents an 

 inexact parallelism, but they are also more distinct than in the mature Speler- 

 pes bellii, where, as might be supposed, the foetal union is delayed to ma- 

 turity in other respects, as in a specimen from Orizava, Mex., of 16-5 1. in 

 length. Here the union is about as extensive as in CEdipus m o r i o. In the 

 young of Thorius pennatulus the digits are not distinguished in specimens 

 of -66, the full size, and otherwise entirely mature. In the adult they are dis- 

 tinct for half their length. The digits in the young larva of Gyrinophilus 

 salmoneus are as distinct as in those of Spelerpes rubra; in one example 

 I find the simple foot of earlier stages retained, resembling exactly that of 

 CEdipus, excepting that there are emarginations for but three toes, instead of 

 five. Genera which have no premaxillary fontanelle at maturity have it in 

 the larval stage. Finally closely allied genera, which only differ in the degrees 

 of ossification of the parietal and palatine bones, represent simply the relation 

 between undeveloped and developed conditions of the same form. 



The relations of the genera may be expressed as follows : Those of the first 

 or Plethodontine section are related to those of the second or Spelerpine, by 

 an inexact parallelism, excepting Anaides, whose peculiarities exclude it from 

 the comparison. Those in each section differing in the union or separation of 

 the premaxillary bones, are related in the same way to each other. The nearly 

 allied genera in the Plethodontine group are Hemidactylium and Plethodon, 

 and Batrachoseps and Stereochila. In the first case we have only inexact 

 parallelism, because while Plethodon has the four digits of Hemidactylium, its 

 parietal bones are unossified, though an acceleration of development in this 

 respect would render the relation one of exact parallelism. This is apparently 

 the relation between Batrachoseps and Stereochilus, for with the foetal digits 

 of the latter, the former preserves also its foetal cranium. It only remains to 

 ascertain whether Stereochilus loses its branchia; before or after acquiring the 

 normal number of digits. From the very small size of one at least of these, 

 the former case seems probable, but I have not yet been able to prove it by 

 direct observation. Should it be so, we would have a case of exact paral- 

 lelism. 



In the Spelerpine group the relation between Geotriton and Gyrinophilus is 

 again one of inexact parallelism, since when the digits of the latter are only sepa- 

 rated at the extremities, as in the former, the animal is still branchiferous and 

 possesses the larval tongue, etc. The same occurs in Spelerpes when certain of its 

 species present but four hind toes as in Manculus; therefore the relation of these 

 two is also of inexact parallelism. The relation of Oplieobatrachus is, however, 

 one of exact parallelism, for its characters are all found in some young Speler- 

 pes at an immature age, subsequent to the absorption of the branchiae. With 

 CEdipus, if the condition be not that of exact parallelism with some species of 

 Spelerpes, the approach to it is close, as above observed. It is chiefly pre- 

 vented by the fact that the ossification of the parietal bones in most species of 

 the latter takes place after the extremities are fully developed. It is to be 

 observed in this connection that, as has been above pointed out, the separation 

 of the digits takes place at very different periods in the history of the different 



[May, 



