44 



THE QUARTERLY REVIEW OF BIOLOGY 



surpassed evolutionary changes in con- 

 temporary land dwelling carnivores. The 

 future will show, I believe, that the 

 geological history of the odontocetes and 

 mysticetes is considerably longer than 

 some investigators have conceded. 



Returning to telescoping as a factor in 

 the remodeling of the cetacean skull, we 

 find that one of the earliest and most 

 obvious results of this general evolution- 

 ary trend was the reduction and virtual 

 elimination of the intertemporal region. 

 This was not accomplished by the elimina- 

 tion of elements in the cranial region, but 

 by the elongation of certain bones and the 

 slipping of others over those with which 

 they at one time met edge to edge in 

 sutural union. In one group, the Odon- 

 toceti, there has been an elongation of 

 the proximal portion of the maxillary and 

 in the other, the Mysticeti, an elongation 

 of the supraoccipital. In the first group 

 there has been a backward overthrust of 

 anterior cranial elements and in the second 

 a forward overthrust of posterior cranial 

 elements. While this general tendency 

 in direction of the thrust prevails in most 

 of the odontocetes under consideration, 

 types also exist in which the backward and 

 forward overthrusts are combined in such 

 a way as to give rise to a more or less 

 balanced condition of telescoping, such 

 as is found in the physeteroids, while in 

 the ziphioids the primary backward max- 

 illary overthrust apparently was stopped 

 abruptly by a final forward occipital 

 thrust. 



There is one fundamental difference in 

 construction between the whalebone and 

 the toothed-whale type of skull, and this 

 feature may account for the different 

 structural modifications that characterize 

 these two living groups of cetaceans. In 

 skulls of whalebone whales (Mysticeti), 

 the maxillary can not overspread the 

 braincase because its posterior extremity 



straddles the supraorbital process of the 

 frontal in the form of two processes, the 

 lower of which is known as the infra- 

 orbital and the upper as the ascending 

 process. The upper surface of the ex- 

 panded supraorbital process of the frontal 

 is left bare and the relatively narrow 

 ascending process of the maxillary is 

 mortised into the body of the frontal 

 behind the level of the narial passages. 

 The infraorbital process of the maxillary 

 lies beneath the anterior border of the 

 inferior surface of the supraorbital process 

 of the frontal and projects conspicuously 

 behind and beneath the infraorbital 

 foramen. It is quite obvious that any 

 backward movement of the maxillary 

 would be retarded by this interlocking 

 with the supraorbital process of the 

 frontal, but the impetus for telescoping 

 was not to be blocked by any such impedi- 

 ment. Inhibited by what appear to be 

 insurmountable obstacles at the base of 

 the rostrum, the direction of the thrust 

 was reversed from backward to forward. 

 This forward overthrust of the elongate 

 occipital shield carried the posterior and 

 intertemporal elements forward, bringing 

 the apex of the supraoccipital to or beyond 

 the median interorbital level and forcing 

 the parietal to override the frontal above 

 the base of the supraorbital process, in 

 some instances as far forward as the level 

 of the proximal end of the nasal (Balaenof- 

 tera acuto-rostrata, B. physalus, and B. 

 borealis). 



In the toothed whale (Odontoceti) 

 type of skull, as heretofore mentioned, 

 the direction of the overthrust of the 

 cranial elements has been mainly toward 

 the rear. The entire proximal portion 

 of the maxillary is pushed back over the 

 supraorbital process of the frontal to 

 meet or approach the supraoccipital at the 

 level of or behind the orbit; laterally this 

 bone spreads out and, together with the 



