208 INTRODUCTION TO EVOLUTION 



The development of a proboscis has a peculiar effect upon the topog- 

 raphy of the skull. In most mammals the external nares, bony openings 

 in the skull at the base of the nostrils, are at or near the anterior tip of 

 the skull. In mammals that develop a proboscis the nares recede from the 

 tip; in elephants they have receded so far up the front of the skull that they 

 appear to be located in the middle of the forehead (Fig. 10.9). Presumably 

 this recession of the nares is connected with the necessity for firm anchor- 



FIG. 10.9. Skull of elephant, sectioned longitudinally. 6, brain 

 cavity. D, diploe (air cells). /, incisor (tusk), m', m^, m', molars. 

 N, nares. (From Lull, Organic Evolution, p. 561. Copyright 1945 

 by R. S. Lull. Used by permission of The Macmillan Company, pub- 

 lishers.) 



age for the powerful musculature of the proboscis. Owing to this effect of 

 proboscis development upon skull topography it is possible to estimate 

 from the structure of the skull the size of proboscis possessed by a 

 prehistoric proboscidean. 



Another characteristic of the elephant skull is development of extensive 

 air cells or diploe (Fig. 10.9). While these serve to lighten the skull their 

 principal function is probably connected with support of the heavy head. 

 It will be noted from the figure that an elephant's "high forehead" is not 

 caused by brain development but by the presence of these diploe. The 



