Article IX. — ADAPTIVE SIGNIFICANCE OF THE 

 SHORTENING OF THE ELEPHANT'S SKULL. 



By W. K. Gregory. 



Plate XXIII and 4 Text Figures. 



The following observations were made in connection with 

 Professor Osborn's forthcoming memoir on American fossil 

 Proboscidea, for which were also prepared the figures here 

 used by his kind permission. 



Weithofer ' regards the changes in the elephant's skull ob- 

 servable during individual growth as well as in phylogeny, 

 such as the compression and vertical heightening and deep- 

 ening of the skull, the wide separation of the inner and outer 

 tabulae of the bones, and the cancellous condition of the diploe, 

 the forward shifting of the orbits from a point above the an- 

 terior grinders, etc., as primarily correlated with the prodi- 

 gious development of the tusks — weapons and crow-bars 

 whose effectiveness increased with and reciprocally hastened 

 the phyletic advance in body dimensions. Now the earliest 

 known proboscideans (McBritherium) possessed upper as well 

 as lower incisor tusks, and there are other grounds also for 

 inferring that the ancestors of Dinotherium probably pos- 

 sessed upper incisor tusks of small size, even smaller than 

 in the primitive Mastodon (Trilophodon) eiihypodon ; the pre- 

 sence of these may have initiated the shortening of the 

 head, but the final compression of the skull in Dinotherium 

 progressed notwithstanding the reduction and entire dis- 

 appearance of upper tusks, and this, together with the 

 underlying similarity of its skull to that of Mastodon and of 

 Elephas, shows that » some other factor must also be repre- 

 sented in the extraordinary end results of the process of fore- 

 and-aft compression. 



This factor seems to be the development of the proboscis. 

 This unique organ probably owes its existence partly to the 

 shortening reach of the head and neck which took place 



^ Die Fossilen Proboscidier des Arnothales in Toskana. 4to, Wien, 1890. 

 [387J 



