HISTORY OF WHALES 



4i 



zeuglodonts were much smaller and repre- 

 sent another line of descent. In later 

 Eocene zeuglodonts the dentition is re- 

 duced by the loss of the third upper molar. 

 The elimination of the postero-internal 

 cusp of the cheek teeth was followed or 

 accompanied by the loss of the third root. 

 In the case of Basilosaurus cetoides of North 

 America as well as "Zeuglodon' isis 

 (Stromer, 1908) of the Fayum the cheek 

 teeth have not more than two roots, and 

 not even a vestige of the postero-internal 

 cusp remains. The cheek teeth have a 

 laterally compressed crown, that of the 

 premolars serrated on both edges and that 

 of the molars on one or both edges. The 

 shoulder blade is cetacean-like, but the 

 humerus has held more closely to the 

 original form, retaining a well developed 

 deltoid ridge, greater and lesser tuberosi- 

 ties, and a hinge-shaped distal articular 

 surface. The sternum is relatively large 

 and is made up of several sections. At 

 least two of the small zeuglodonts, 

 Zygorhiza minor and Proieuglodon stromeri, 

 have lumbar vertebrae with short centra 

 and long articulating zygapophyses, while 

 the large Basilosaurus cetoides and " Zeuglo~ 

 don' isis have lumbars with greatly elon- 

 gated centra, although the neural arches 

 have remained short. This specialization 

 is unique among mammals, for the greater 

 part of the vertebral column is articulated 

 only by the centra, the zygapophyses 

 being separated by too wide an interval to 

 function in the normal manner. 



The short bodied zeuglodonts, such as 

 Zygorhi%a minor, which have a short neck, 

 thoracic and lumbar vertebrae with high, 

 broad neural spines, well developed trans- 

 verse processes, and articulating zygapo- 

 physes, must have had a propelling 

 mechanism of considerable strength, for 

 among living cetaceans the most powerful 

 swimmers have a vertebral column of this 

 type. The conclusion can be drawn that 



these short bodied zeuglodonts propelled 

 themselves by upward and downward 

 strokes of the flukes, after the manner of 

 living whales. They were therefore un- 

 doubtedly powerful swimmers, capable of 

 diving or turning at will, and were 

 equipped with a dentition admirably 

 fitted for seizing and holding their prey. 



Conversely the gigantic zeuglodonts, 

 such as Basilosaurus cetoides, which have 

 an elongated trunk and tail, the neck 

 conspicuously shortened, thoracic and 

 lumbar vertebrae with low neural spines 

 and short transverse processes dispropor- 

 tionate to the length of the skeleton, and 

 zygapophyses of all the trunk vertebrae 

 with the exception of the first four or five 

 dorsals, separated by increasing intervals 

 proportional to length of centrum, could 

 hardly have had a powerful propelling tail 

 operating in the same fashion. On the 

 contrary these details indicate that the 

 tail was adapted for lashing, and that in 

 swimming the animal progressed by 

 marked sinuous or serpentine movements. 



By the close of the Eocene period zeu- 

 glodonts had spread widely, for their 

 remains have been found in Russia, Eng- 

 land, and other localities in Europe, the 

 Atlantic and Gulf Coastal plains of the 

 United States, New Zealand, and Seymour 

 Island in the Antarctic. A few of the 

 smaller forms held over until near the close 

 of theOligocene. 



TYPICAL CETACEA 



The conclusion that cetaceans living 

 and extinct have arisen from a multiserial 

 stock appears to be borne out not only by 

 osteological, but also by anatomical evi- 

 dence. That at least three distinct lines 

 of descent are represented among the 

 known Cetacea is evidenced by the general 

 acceptance among mammalogists of the 

 three suborders, of which the Archaeoceti 

 or zeuglodonts at present are known to 



