250 BULLETIN 34, UNITED STATES NATIONAL MUSEUM. 





Arciferi. 



Rana. 



Pseudis. 



Hoplobatrachus. 



Myxopbyes. 



Pysicephalus, 



Ceratophrys. 



Leptopelis. 



Hyla. 



Hyperolius. 



Hylella. 



Heteroglossa. 



Acris. 



Cassiua. 



Cystiguathns 



Hemimantis. 



Paludicola. 



Ill strict refereuce to tlie extension of the webs tlie following parallels 

 may be drawn : 



External metatarsal free : 



Aquatic. 



Snbfossorial. 

 External metatarsal attached : 

 Feet webbed — 



Burrowing. 



Arboreal (vora. teeth). 



Arboreal (uo vom. teeth), 



Aquatic. 

 Feet not webbed — 



Terrestrial. 



Terrestrial, spurred. 



It is, however, remarkable that the raniform tree-frogs nearly always 

 have the external metatarsal bone free; the arciferous always bound. 

 The terminal phalanges of the latter are constructed on a ball-and claw 

 type. In the former the}^ are X-shaped or bifurcate, except in the single 

 West African genus Leptopelis. where the South American type is re- 

 peated. 



I have also discovered another series of parallels which the genera 

 of most of the families of the Salientia present, in the degree of ossifi- 

 cation of the superior cranial walls.* In the least-ossified crania we 

 find the superior part of the ethmoid still cartilaginous, the superior 

 wall of the brain-case membranous, and the prefrontals represented by 

 narrow lateral splints of bone. In genera of slightly advanced type 

 the roof of the ethmoid is ossified, and the prefrontals are wider. In 

 better-developed genera the frontoparietal bones ossify and close the 

 foutanelle. The higher ossification shows itself in an exostosis of the 

 superior cranial walls, which, in further stages, involves the skin, so 

 that it is no longer free from the cranium. The next stage roofs over 

 the temporal muscle with bone, and the highest stage, known only in 

 a genus of BufonidiE (Otaspis Cope), incloses the membranum tympani 

 behind. The following table expresses these facts. 



These series give an excellent illustration of the development of a 

 single character independently of other characters, and show how the 

 generic characters originate quite independently of all others. 



*See Nat. Hist. Eev., 1865; Proceed. Ac. Phila., 1868 (on the Origin of Genera); 

 Origin of the Fittest, 1887, p. 218, Plates iv and v. 



