RULERS OF THE ANCIENT SEAS 69 



heds^ specimens may he seen in almost any collection. In 

 the United States National Museum^ the jaws of a ttcelve- 

 foot blue shark are shozcn for comparison. The largest 

 tooth in that collection is 5f inches high and 5 inches 

 across the base. It takes fve teeth of the blue shark to 

 fll the same number of inches. 



The Mosasaurs are described in detail by Professor S. 

 W. Williston^ in Vol. IV. of the ^^ University Geological 

 Survey of Kansas.'''' There is a technical — and, conse- 

 quently, im'interesting — account of Zeuglodon in Vol. 

 XXIII. of the " Proceedings of the United States Na- 

 tional Museum,^'' page 327. 



Fig. 12. — A Tooth of Zeuglodon, one of the " Yoke 

 Teeth," from which it derives the name. 



