NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 45 



The Hysenodonts, the nearest stock-forms of the cetacea among 

 terrestrial mammals, lived at the beginning of Tertiary times in the 

 northern parts of both the Old and New Worlds. They had spread 

 over Europe and North America and were found in northern Africa 

 as well. The whales must have made their appearance somewhere 

 within the territory occupied by the Hysenodonts, and probably in 

 the oldest part of the Tertiary ; in agreement with this the most 

 primitive cetacean that is yet known, the Hysenodont-like Protocctiis 

 of the family Zeuglodontidae, is found in Egypt in Eocene strata. 

 Likewise one of the next links in the chain of cetacean development, 

 Prozeuglodon, was Egyptian, from the Eocene. But soon the mem- 

 bers of the family must have spread widely ; in any event the highest 

 genus, the almost fantastic, snake-like Zcuglodon [Basilosaitrus], 

 appears to have found its way during the Eocene to all oceans. 



The Zeuglodonts died out early in the Tertiary. Their highest 

 forms left no descendants ; but from the more primitive genera of 

 the family sprang the new family Balasnidas. The oldest, tooth- 

 bearing forms of Balaenids are as yet scarcely known. In Miocene 

 times, however, the family had already produced the specialized 

 whalebone-bearing forms, a side branch on the cetacean genealogical 

 tree, and they soon spread themselves to all the seas of the globe, 

 where they still are found. Some of the recent genera are essentially 

 cosmopolitan, even in the sense that individual species occur in all 

 seas. This holds good in part only of Balana, one of whose species, 

 the more primitive, B. australis, is almost cosmopolitan, while the 

 second, the more specialized, B. mysticetus, is confined to the northern 

 polar oceans. It is literally true of Balccnoptcra and Mcgapfcra. 

 Two of the recent genera are confined to a smaller range : Ncobalccna 

 a relatively high genus that lives in the South Sea, where it likely 

 originated, and Rhachionectes a relatively low genus, in many respects 

 recalling extinct Miocene forms. It lives in the northern part of the 

 Pacific, perhaps as a kind of last remnant from an early day. The 

 reason why the Balsenids, in spite of their rather primitive structure, 

 are not wholly extinct, supplanted by the more specialized cetacea, 

 is probably because they have chosen a peculiar food supply: the 

 small creatures of the sea. Therefore they do not have very many 

 competitors among their kind. 



From the most primitive, tooth-bearing Balsenids the family 

 Squalodontidae branched off in Tertiary times. It had its flourish- 

 ing period in the Miocene, widely distributed in the oceans. The 

 whole family disappeared before the end of the Tertiary, chiefly, 

 it would appear, because it passed onward into its successors. 



