REVIEW OF THE MODERN DOCTRINE OF EVOLUTION. 217 



Fig. 8. 



Fig. 9. 



ions, which differ only in the structure of the lower portion of 

 their scapular arch, or shoulder-girdle. In the one the opposite 

 halves are capable of movements which contract or expand the 

 capacity of the thorax; in the other the opposite halves abut 

 against each other so as to be incapable of movement, thus pre- 

 serving the size of the thoracic cavity. But during the early 

 stages, the frogs of this division have the movable shoulder-girdle 

 which characterizes those of the 

 other division, the consolidation con- 

 stituting a modification superadded 

 in attaining maturity. Further- 

 more, young Amir a are toothless, 

 and one section of the species with 

 embryonic shoulder-girdle never ac- 

 quire teeth. So here we have a 

 group which is imperfect in two 

 points instead of one. This is the 

 tribe Bufoniformia; the tribe with 

 teeth and embryonic shoulder-girdle 

 is called the Arcifera, and that 

 which is advanced in both these res- 

 pects is the Raniformia. Now the 

 frogs of each of these divisions pre- 

 sent nearly similar scales of develop- 

 ment 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 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 the sixth 

 form the ossification is extended so as to roof in the temporal 

 muscles and inclose the orbits behind, while in the rare seventh 



Fig. 10. 



SlIOULDEK-GIRDLES OF " AnURA." 



Fig. 8, of the Arciferous type 

 (Scaphiopus holbroohi). Fig. 9, 

 Rana temporaria^ tadpole -nith 

 budding limbs. Fig. 10, do. adult. 

 Figs. 9 and 10 from Parker. 



