xii. 14 EVOLUTION OF THE SKULL 331 



They run from the hind end of the jaw to the surface of the skull and 

 squamosal. 



The skull and jaws of the frog thus constitute a protection for the 

 brain and special sense-organs, a feeding apparatus, and a means of 

 respiration. The heavy protection afforded by the dermal bones of 

 fishes and early amphibians has been largely dispensed with, probably 

 for lightness. The front part of the skull, concerned with the nose, 

 eyes, and brain, has become increased in size and the hind part, 

 originally concerned with the gills and pharynx, greatly reduced. 



dep.mand. 



Fa se. dors, 

 iat. dors. 



cor rad 

 FL. c uin. 



Fig. 197. Muscles of head and neck of frog dissected from the side. 

 add. adductor mandibulae. Other letters as in Figs. 187 and 191. (Partly after Gaupp.) 



These changes, carried to extremes in frogs, have been in progress 

 throughout the evolution of amphibia. It is not difficult to imagine 

 that they have been the result of rather simple genetic changes, affect- 

 ing the relative growth of various parts of the skull. We are still far 

 from the knowledge necessary to say exactly what developmental 

 changes have occurred, but we know enough to imagine how selection 

 through millions of years has changed the quantities of certain sub- 

 stances so as to produce gradually less bony and shorter heads, such 

 as enabled their possessors to maintain sufficient mobility to hold a 

 place in a world peopled by the reptiles and other still more active 

 descendants of the early amphibians. 



The preceding account, particularly with regard to the osteology, 

 should not be regarded as diagnostic of all anurans. Bufonid skulls 

 are completely devoid of teeth but they possess a supratemporal bone, 

 which fuses with the squamosal and roofs the otic capsule. Hylids 

 frequently develop secondary dermal ossifications to form expanded 

 helmets; this trend also occurs in leptodactylids (e.g. Calyptocephalus), 

 where the skull may be so completely roofed and sculptured as 

 to simulate the condition of the extinct branchiosaurs. Pseudoteeth 



