ON THE ORIGIN OF GENERA. 47 



cephalus, wliicli differs in the normal exostosis of the cranium 

 not involving the derm, as in the former. Close to this is 

 Scytopis, where the fully ossified cranium is not covered by an 

 exostosis. Next below Scytopis is Hyla, where the upper surface 

 of the cranium is not ossified at all, but is a membranous roof 

 over a great fontanelle. Still more imperfect is Hylella,* which 

 differs from Hyla in the absence of vomerine teeth. Now, the 

 genus Trachycephalus, after losing its tail and branchiae, pos- 

 sesses all the characters possessed by the genera Hylella and Hyla, 

 either at or just before the mature state of the latter, as the 

 ethmoid bone is not always ossified in advance of the parietals. 

 It soon, however, becomes a Scytopis, next an Osteocephalus, 

 and finally a Trachycephalus. It belongs successively to these 

 genera, for an exhaustive anatomical examination has failed to 

 reveal any characters by which, during these stages, it could be 

 distinguished from these genera. 



Now, it would be a false comparison to say that the young of 

 Trachycephalus was identical with the genus Agalychnis, which 

 in truth it resembles, because that genus is furnished with one 

 other character — the presence of a vertical pupil — and belongs to 

 another series in consequence, which is represented as yet, with 

 our present imperfect knowledge — or perhaps imperfect fauna — 

 by three genera only. 



2. The lowest type of the near allies of our common fresh- 

 water frogs is the genus Eanula, where the prefrontal bones are 

 narrow strips on each side the ethmoid cartilage ; the ethmoid 

 cartilage itself entirely unossified above, and the vomerine teeth 

 very few and on a small elevation. There are two species, R. 

 affinis and R. palmipes.\ The other species have the ethmoid 

 cartilage ossified above, at least beneath the extremities of the 

 frontoparietals. 



Those of the latter most like Eanula possess the same type of 

 narrow prefrontals, separated by a broad area of cartilaginous 

 ethmoid, and fasciculi of teeth. Of this type is Rana delalandii, 

 and probably R. porosissima, Steind., of the South Ethiopian 

 region. Other species of the same type extend their vomerine 

 patches into lines ; such are R. mascariensis, R. fasciata, R. 

 oxyrhynchus, R. grayi, and other South African species. 



* I refer to H. carnea m., not having Reinhardt and Liitken's type of this genus, 

 f These species are now believed to be identical, but three others have been dis- 

 covered (1886). 



