NO. 5 TELESCOPING OF THE CETACEAN SKULL I7 



flattening the posterior border of the latter bone until in the region of 

 the ascending process there is nothing more than a broad postero- 

 superior angle. These peculiarities may be due in part to the arching 

 which the skull has undergone in connection with the development 

 of the greatly elongated plates of baleen within the mouth cavity ; 

 but, whatever their origin, they appear to have resulted in modifica- 

 tion almost exclusively through forward overthrust and movement 

 from behind. Of the genera in which this kind of telescoping has 

 taken place Bahcna and Enhalccna present the extreme of specializa- 

 tion in the development of the food-straining apparatus, a specializa- 

 tion which involves increase in the size of the head as compared with 

 that of the body, and a great upward-arching of the whole skull ac- 

 companied by lengthening of the baleen plates and enlarging of the 

 suspension area for the lower jaw. In Ncohalcena on the other hand 

 the skull is moderately arched, and the form of the rostrum is 

 peculiar among the living baleen whales ; broad at the base and 

 narrowing rapidly to an attenuate tip ; mandible excessively robust, 

 strongly bowed outward. The rib-bearing portion of the vertebral 

 column in Neobalcena shows the maximum condition of development 

 known in the group, but the cervicals are completely fused with each 

 other, and the hinder part of the column is so remarkably reduced 

 by suppression of some of the caudals and all but two of the lumbars 

 that the number of vertebrae behind the dorsals does not exceed that 

 of the dorsals themselves, while in all other baleen whales it is at 

 least twice that of the dorsals. The ribs are large and broad, but all 

 except three or four at the anterior end of the series are unattached 

 to the vertebras, a condition not known in other whalebone whales. 

 These characters of Ncohalcena indicate such a fundamental diver- 

 gence on the part of this genus from Balana and Enhalccna that it 

 should be placed in a separate family. 



In all of the other mysticete genera except the extinct near relatives 

 of the balsenids the backward movement of the anterior elements of 

 the skull appears to have been an important part of the process of 

 telescoping. The nasal bones and the nasal branches of the pre- 

 maxillaries are no longer situated obviously and conspicuously out 

 in the rostrum. Their bases are forced back to or beyond the level 

 of the anterior border of the supraorbital portion of the frontal, thus 

 finally reaching a position which they do not occupy in any known 

 land mammals and which gives them the appearance of belonging to 

 the facial part of the skull. 



