98 THE NATTJRAL niSTCfRY EEVIEW. 



burrowing, others for an aquatic life ; some are entirely terrestrial, 

 and some are constructed for maintaining their position upon the 

 leaves and branches of trees. But the different adaptive modifica- 

 tion so graduate into each other on one hand, while similar ones are 

 so constantly separate on the other, different structures frequently 

 serving the same purpose,* that we are compelled to believe that a 

 different idea pervades the scheme ; and that, although adaptive 

 modifications undoubtedly distinguish many generic and such subor- 

 dinate types, the direction of their series is in accordance with 

 another law which is not explained. This is the case, it will be seen, 

 within the more definitely restricted series, the families. 



In addition to the many species constituting the three suborders 

 above-mentioned, there are known three living and perhaps as many 

 extinct ones characterized by an extension of the pterygoid bones so 

 as to enclose the cava tympani and tubse Eustachii, causing the latter 

 to present a single united ostium pharyngium. The living species 

 have at the same time no tongue. The genera Fipa, Dactylethra, 

 and Palceohatrachus are alluded to. The vertebrs3 in these animals 

 are opisthocoelian, as in the Salamanders, and their sternum of the 

 arciferous type. 



With our present knowledge these types may be regarded as 

 constituting a distinct suborder ; but it is possible that Palaolatra- 

 chus and Dactyletlira may come to be looked upon as extremes of the 

 series of Arcifera, succeeding the family Asterophrydidcs of the 

 latter. The peculiar vertebras without ribs and the simply arti- 

 culated coccyx are points of resemblance w^hich do not occur 

 elsewhere. In Pipa the relations of the fronto-parietal, ethmoid 

 and prefrontal bones, also the sternum, find a close parallel in the 

 BTiinopJirynidce, which, with the absence of teeth, suggest that it 

 may be the most divergent type of the Bufoniform suborder. 



AGLOSSA. 



PlPID^. 



No ribs ; simple coccyx attached to a single condyle. Coracoid 

 and epicoracoid divergent, their connecting arches not overlapping. 

 No manubrium. Pronto-parietal completely ossified; prefrontals 



* Vide Professor Owen, in Trans. Zool. Soc. vol. v. p. 91, line 21. 



