I 



36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



in the main, as if it had consisted of three loosely joined masses of 

 plastic material arranged serially and acted upon by two opposing, 

 compressing, forces. One of these forces would have pushed for- 

 ward from behind, the other would have resisted or pushed backward 

 from in front, with the result that forward motion was established 

 in the posterior mass and backward motion was established in the 

 anterior mass, these two masses acting at the expense of the one 

 which lay between them, covering it, eating into it, and finally ob- 

 'iterating a great part of it (compare the large frontal and parietal 

 in a normal skull, pi. 7, fig. i, with the completely eliminated parietal 

 and excessively modified frontal in Kogia, pi. 7, fig. 3). Of the 

 three masses the anterior would be represented by the rostrum, the 

 posterior by the occipitals and squamosal, the intermediate by the 

 frontal, parietal and sphenoids. While motion of the cranial elements 

 has occurred in every known modern cetacean the dominant direc- 

 tion of this motion has not always been the same ; sometimes it has 

 been chiefly a forward thrust of the posterior elements (pi i, fig. 2; 

 pi. 6, fig. 4), sometimes most conspicuously a backward thrust of the 

 anterior elements (pi. i, fig. i ; pi. 7, fig. 2), occasionally a less one- 

 sided combination of the two thrusts (pi. 7, fig. 3; pi. 8, figs. 3, 4). 

 The causes which actually determined these difl:'erences in the mo- 

 tions of the various bones are now unknown ; but the special peculiari- 

 ties in the behavior of the moving cranial elements in each of the two 

 main types seem to have been predominantly such as might be 

 imagined to have resulted from the encountering of definite mechani- 

 cal obstacles, varying in their degree of efficiency, lying some of them 

 in the region of juncture between the occipital and the parietal, others 

 in that between the maxillary and the frontal. 



The possible character of the obstructions to the telescoping pro- 

 cess, and the reason why these obstructions, instead of being uniform 

 in all cetaceans, were more efifective checks to movement at one point 

 in the mysticetes and at another in the odontocetes, can be surmised 

 from the rather unexpectedly conspicuous differences which have 

 been found to exist in the details of the devices by which the bones 

 of the skull are interlocked in mammals whose heads have not been 

 subjected to compression (pp. 5-6; pis. 2-3). As to the nature of the 

 compressing forces : the one acting from behind would probably be 

 represented mostly by the mere forward push of the rapidly swim- 

 ming body ; that acting from the front would be the resistance of the 

 water. In the toothed cetaceans the modifications seem to have pro- 



