ON THE ORIGIN OF GENERA. 53 



European, North American, 



1. Temporal fossa over arched. 



Oultripes. * * 



Temporal roof not ossified. 



2. Fronto-parietal bones ossified, involving 



derm. 



Pelobates. Scapliiopns. 



3. Fronto-parietals ossified, distinct from 



derm. 



* * (Unknown.) * * 



4. Fronto-parietals not ossified, distinct 



from derm. 



Didocus. Spea. 



In this case Didocus can not be said to be identical as a genus 

 with an undeveloped stage of Oultripes, since, while the cranium 

 of the latter is in the condition of Didocus, it bears a long tail, 

 and the limbs are but little developed. Nor is Didocus identical 

 with the undeveloped condition of Pelobates, since both cranium 

 and limbs of the latter are developed before the tail is absorbed. 

 Nor is Pelobates identical with the undeveloped condition of Oul- 

 tripes, since, while the cranium of the latter is that of the former, 

 the limbs and tail are still larval. The same relations exist be- 

 tween the other members of the family; The genus Scaphiopus 

 is not an undeveloped form of Pelobates as to its auditory organs, 

 for, when the latter is identical with the former in this respect, it 

 bears otherwise entirely larval characters. Nor is Spea an arrested 

 Scaphiopus, the relation being here precisely that between Dido- 

 cus and Pelobates. Spea approaches more closely ah arrested 

 Didocus in all respects, but that when the latter possesses the au- 

 ditory apparatus * of the former, it is a larva in limbs and tail, 

 and that it loses this apparatus before reaching the other charac- 

 ters of Spea. The relations of these genera, as compared with 

 those of the Trachycephalus, Oystignathidse, and Bufo series, may 

 be represented as follows : the lines represent the developmental 

 scale of each. 



This is an example of the simplest case of inexact parallelism, 

 as distinguished from the exact parallelism or identity. As the 

 fauna of the present period is but a fragment, so the simple inex- 



* The possession of eavum tympani and tuba Eustachii in the undeveloped con- 

 dition of this genus is only assumed from its close relation to Pelobates. 



