48 GENERAL EVOLUTION. 



The prefrontals are subtriangular, and approacli each other 

 more or less in the numerous species of North America and of 

 the Eegio Palsearctica, while generally the vomerine teeth are in 

 fascicles or very short series. In the Ethiopian Rmia fuscigula 

 the prefrontals unite on the median line, roofing over the ethmoid 

 cartilage and reducing it, while the vomerine teeth are in very 

 short lines. 



In the species of the Palseotropical region, Rana tigrina, R. 

 vittigera, R. cyanoplilyctis, R. grunnietis, R. hexadactyla, R. 

 corrugata, R. elirenhergii, R. gracilis, and the Ethiopian R. occi- 

 pitalis, the prefrontals not only unite solidly (the suture remain- 

 ing on the median line), but extend and closely fit to the fronto- 

 parietals. The vomerine patches have lengthened out into series. 



Now, the young of the latter type of Rana (I take as an ex- 

 ample the R. tigrina, one of the most abundant and largest of 

 Indian frogs) presents the subtriangular prefrontals neither in 

 contact with each other or with the frontoparietals, and the 

 vomerine series is much reduced ; in fact, it belongs in all 

 respects to the Palsearctic group. I have not examined younger 

 specimens, but have no doubt they are like those of the Palae- 

 arctic ; which are, in their young stage, precisely of the type 

 of the -Ethiopian Rana, with fasciculate teeth like the young of 

 those of the same region with teetli in series, since the prefrontals 

 are still more reduced, becoming linear. Finally, the first stage 

 of the Nearctic Rana, after losing the larval tail, is the genus 

 Ranula, having linear prefrontals, minute vomerine teeth, and 

 the ethmoid ring cartilaginous above. 



These points of structure are of generic quality, but I have 

 not regarded any group as sufficiently defined to be so regarded, 

 except Ranula, as the adults of some species appear not to be con- 

 stant in possessing them. Thus a very large Rana catesheyana 

 sometimes exhibits prefrontals in contact on the median line, 

 while it is difficult to say whether R. areolata of North America 

 is of the Nearctic type so much as of the ^Ethiopian. Neverthe- 

 less, the groups are generally quite geographically restricted. 



3. A similar relation exists between the genera Hyperolius, 

 Staurois, and Heteroglossa in respect to the prefrontal bones and 

 the separation of the outer metatarsi, and — 



4. Between Ixalus, Rhacophorus, and Polypedates also, in 

 reference to vomerine teeth, bifurcation of last phalange, and 

 dermoossification of the cranium. 



