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THE QUARTERLY REVIEW OF BIOLOGY 



have been in existence at a more remote 

 time than the Odontoceti or toothed 

 whales; while the known geological 

 history of the Mysticeti or whalebone 

 whales is much shorter and does not date 

 at present farther back than the Oligocene. 

 The exigencies of an aquatic mode of 

 living are such that the members of the 

 several families of these three suborders 

 assumed a similar outward appearance in 

 conformity with streamline requirements 

 such as have been forced upon many other 

 unrelated groups of aquatic vertebrates, 

 but there is no palaeontological evidence 

 to support any assumption of a remodeling 

 of originally unlike parts to a similar 

 adaptive use. The directions in which 

 the members of these three suborders, 

 collectively and individually, subse- 

 quently developed were predetermined by 

 peculiarities of structure in existence at or 

 before the entrance upon a pelagic habitat. 

 The resemblances between these three 

 suborders are such as to suggest a rather 

 close relationship of the original ancestral 

 forms and common descent from some 

 broad headed ancestral stock in which 

 the critical cranial elements were united 

 by squamous instead of dentate sutures. 

 No extinct cetacean skull known holds 

 proof of descent of an archaic toothed 

 whale from any known zeuglodont, or 

 presents structural conditions necessary 

 to form the basis for the elaboration of 

 the mysticete and odontocete types of 

 skull from any common cetacean stock. 



A mammalian skull is composed of a 

 number of distinct bones, some of which 

 build up a closed box to protect the nerv- 

 ous system; on the lower posterior bones 

 of the skull are located the articular sur- 

 faces for the supporting atlas vertebra of 

 the neck and the bones of the tongue, 

 while others make up the jaws and the 

 muzzle portions. The exposed surfaces 

 of the several bones give origin to 



or afford insertion for the various muscles 

 which are found in the region of the head. 

 In the generalized type of skull, such as 

 may be found in some of the early Tertiary 

 land mammals, the individual bones 

 exhibit certain mutual contact relation- 

 ships, and this original architectural 

 plan has been retained with few exceptions 

 during the Cenozoic era by most orders of 

 mammals. Changes in the relative size 

 and shape of the individual bones occur 

 frequently in the geological history of 

 most groups, but there has been very little 

 rearrangement of the bones themselves. 

 Skulls of cetaceans other than zeuglo- 

 donts differ from those of all land mam- 

 mals in that they exhibit an extreme type 

 of remodeling and present alterations 

 which have affected not only the relative 

 size and shape of many of the individual 

 bones, but also their mutual relations. 

 Except for the zeuglodonts, the skulls 

 of all known cetaceans show in some 

 degree the effects of telescoping — that is, 

 the braincase or the portion of the skull 

 behind the rostrum has been shortened, 

 mainly by the slipping of one bone over 

 another or by interdigitation. Gerrit S. 

 Miller (19Z3) has discussed the telescop- 

 ing of the cetacean skull in considerable 

 detail and has shown that this process 

 proceeded according to two different plans, 

 one of which is found in the whalebone 

 whales (Mysticeti) and the other in the 

 toothed whales (Odontoceti). 



The departure from the generalized type 

 of land mammal skull is most striking in 

 living cetaceans, but even in the earliest 

 known extinct genera of true whales 

 (Agorophhis ; Xenorophus, and Archaeodelpbis) 

 the telescoping was well advanced. If 

 the zeuglodonts and the archaic toothed 

 whales did arise from terrestrial mammals 

 during the Lower Eocene stage, then the 

 rapidity with which they became so 

 completely adapted for an aquatic life far 



