172 INTRODUCTION TO EVOLUTION 



The lower jaw of reptiles consists of several bones fastened together, 

 whereas the mammalian lower jaw consists of a single pair of bones, the 

 right and left dentaries. In therapsids the dentaries formed most of the 

 jaw, the other bones being reduced in size (Fig. 8.25). The secondary or 

 "hard" palate separating the mouth cavity from the nasal cavity above it 

 was present in therapsids, as it is in mammals. The limbs were developed 

 for better locomotion than characterized other reptiles, the body being 

 lifted higher from the ground and the limbs being placed more directly 

 beneath, instead of spread out at the sides as were the limbs of other 

 Permian reptiles. 



In short, the Permian therapsids were acquiring the characteristics 

 which were to prove of such value to their descendants, the mammals. 



References and Suggested Readings 



Briggs, M. H. "Dating the origin of life on earth," Evolution, 13 (1959), 416- 

 418. 



Colbert, E. H. Evolution of the Vertebrates. New York: John Wiley & Sons, Inc., 

 1955. 



Dunbar, C. O. Historical Geology. New York: John Wiley & Sons, Inc., 1949. 



Hussey, R. C. Historical Geology, 2nd ed. New York: McGraw-Hill Book Com- 

 pany, Inc., 1947. 



Millet^ J. "New facts about coelacanths," Nature, 174 (1954), 426-427. 



Millot, J. "First observations on a living coelacanth," Nature, 175 (1955), 

 362-363. 



Moore, R. C. Introduction to Historical Geology, 2nd ed. New York: McGraw- 

 Hill Book Company, Inc., 1958. 



Raymond, P. E. Prehistoric Life. Cambridge, Mass. : Harvard University Press, 

 1939. 



Romer, A. S. Vertebrate Paleontology, 2nd ed. Chicago: University of Chicago 

 Press, 1945. 



Romer, A. S. The Vertebrate Story. Chicago: University of Chicago Press, 

 1959. 



Simpson, G. G. The Meaning of Evolution. New Haven: Yale University Press, 

 1949. 



Smith, J. L. B. The Search Beneath the Sea. The Story of the Coelacanth. New 

 York: Henry Holt & Co., 1956. 



