MAMMALIAN DAWN. 77 



mentary way a few of the earlier types which have been 

 studied by Leidy, Cope, Marsh-, Osborn and others in their 

 relation to those of the present day. 



In Eocene times, the earliest period of this age, there 

 prowled in the waters along the southern coast of our conti- 

 nent a huge shark, Carcharodon ?fiegalodon, the specific name 

 being derived from the great size of its triangular teeth, 

 which are in some cases as large as a man's hand. The jaws 

 of this monster were four to six feet across, and its length 

 was probably from fift\^ to seventy feet. Sharing in the rule 

 of the waters with this shark was the Zetiglodon (ZevyX-q, a 

 yoke; oSows, tooth), another predaceous creature of great 

 size, deriving its name from a peculiarity of the teeth, each 

 molar being composed of two separate fangs united by a 

 crown, while conical teeth were set in the front of the long 

 and narrow jaws. As the vertebrae of this animal were some 

 of them eighteen inches long and twelve inches in diameter, 

 and as nearly seventy feet of the vertebral column have been 

 found in place, the entire bodj^-length must have been more 

 than sevent}^ feet. These are remains of whale-like propor- 

 tions, and, although they have been numerously found in the 

 Southern Eocene, and it is probable that their range was 

 quite extensive, none has as yet been discovered which might 

 give evidence of a transformation of these primitive cetaceans 

 linking them more closely to our modern whales, or which 

 would connect them with any known form of the Age of 

 Reptiles. Going back of this stock, similarities in the form 

 of the body suggest an alliance with the ichthysaurs, but the 

 history of our marine mammals must be constructed upon a 

 more reliable foundation than that of similarity of body form. 

 These aberrant mammals cannot be brought into a single 

 group, but belong to series not closely related. It is probable 

 that the whales, the sea-cows and the seals originated from 

 very old and separate land-dwelling mammals, the different 

 lira^iches becoming adapted independently for an aquatic life. 

 We recognize here an example of the phenomena of converg- 



