BATRACHTA. 



319 



Fig. 188.— sternum 

 of Scaphiopus hot- 

 hrookii; arcif erous 

 type. 



in two points instead of one. This is the family of toads, or Bufonidae. Several fami- 

 lies have teeth and embryonic shoulder girdle. The family which has teeth and firmi- 

 sternal shoulder-girdle is the Ranidse, or true frogs. Now the frogs 

 of each of these divisions present nearly similar scales of development 

 of another part of the skeleton, viz. : the bones of the top of the skull. 

 We find some in which the front one of these bones (ethmoid) is 

 represented by cartilage only, and the middle ones, or fronto-parietals 

 and the nasals, are represented by only a narrow strip of bone each. 

 In the next type the ethmoid is ossified ; in the next, we have the 

 fronto-parietal completely ossified. The nasals may range from 

 narrow strips to complete roofs. In the fourth station on the line, 

 these bones are rough, with a hyperostosis of their surfaces ; and in the next 

 set of species, this ossification fills the skin, which is thus no longer separable from the 

 cranial bones; in the sixth form the ossification is ex- 

 tended so as to roof in the temporal muscles and enclose 

 the orbits behind ; while in the rare seventh and last 

 stage, the tympanum is also enclosed behind by bone. 

 Now all of these types are not found in all of the families 

 of the Anura, but the greater number of them are. Six 

 pi-incipal families, five of which belong to the Arcifera, 

 are named in the diagram below, and three or four 

 others might have been added. I do not give the 

 names of the genera which are defined as above de- 

 scribed, referring to the explanation of the cuts for 

 them, but indicate them by the numbers on the left margin of the page, which 

 correspond to those of the definitions above given. A zero mark signifies the absence 

 or non-discovery of a generic type. See pp. 320, 321. 



Sternum embryonic. Sternum complete. 



Arcifera. Firmisternia. 



Fig. 189, — Sternum of Eana tempo- 

 raria; firmisternous type. 



Stages. 



No teeth. 



Bufonidse. 



Teeth. 



HylidiE. 



Eanidae. 



1— 



2— 

 3— 



4— 

 5— 

 6— 



7— 



Scaphiopidas Cystignathidse. 

 and Pelobatidae. 



1 



2 2 



3 



4 4 



5 



6 6 

 



It is evident, from what has preceded, that a perfecting of the shoulder-girdle in 

 any of the species of the Arciferous columns would 

 place it in the series of Firmisternia. An accession of 

 teeth in a species of the division Bufonidae would 

 make it one of the Scaphiopidse ; while a small amount 

 of change in the ossification of the bones of the skull 

 would transfer a species from one to another of the 

 generic stations represented by the numbers of the columns from one to seven. 

 There are few groups where this law of parallelism is so readily observed among 



Fig. 190. — Sternum of a larva of 

 Itana temporaria^ ■with the limbs 

 just budding. 



