64 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



the species with embryonic shoulder-girdle never ac- 

 quire teeth. The suborder with embryonic shoulder- 

 girdle is called the Arcifera, and that which is ad- 

 vanced in this respect is the Firmisternia. 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 one of these bones (ethmoid) is represented 



Fig. 17- 



Fig. 16. 



SHOULDER-GIRDLES OF "ANURA." 



Fig. 15. Of the Arciferous type (Phyllomedusa bicolor}. Fig. 16, Rana 

 temporaria, tadpole with budding limbs. Fig. 17, do., adult. Figs. 16 and 17 

 from Parker. 



by cartilage only, and the frontoparietals and 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 frontoparietal completely ossified, and the 

 nasals 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 



