3IO MOSTLY MAMMALS 



In a fossil porpoise {Delphmopsis freyerz) from the middle 

 Tertiary deposits of Radoboj, in Croatia, the tubercles are 

 still more strongly developed, and form a series of regu- 

 larly arranged and parallel rows in the neighbourhood of 

 the back-fin. They clearly indicate one step from the 

 modern porpoises in the direction of a species provided 

 with a functional bony armour in this region of the body. 

 Between the extinct Croatian porpoise and the much more 

 ancient whale known as Zeuglodon, some parts of whose 

 body are believed to have been protected by a bony armour 

 as solid as that of the giant relatives of the armadillos, the 

 intermediate links are at present unknown, although they 

 may turn up any day. Zeuglodon was first discovered 

 in the early Tertiary strata of the United States, but its 

 remains have subsequently been found in the equivalent 

 deposits of Egypt and elsewhere, and in early times it 

 was probably the dominant cetacean of the world. Years 

 ago there were discovered with the bones of the internal 

 skeleton of this whale a number of bony plates which 

 originally formed a dermal armour ; but these plates were 

 regarded as belonging to a species of leathery turtle and 

 as having nothing to do with the whale. 



In microscopic structure, as well as in their arrangement, 

 these polygonal bony plates are said, however, to differ from 

 the armour of the leathery turtle ; while their structure is 

 generally similar to the undoubted bones of Zeuglodon 

 with which they are found in association. Moreover, a 

 fragment covered on one side with armour of this type has 

 been discovered which cannot apparently be any part of 

 the shell of a turtle, but which may well be the back-fin 

 of Zeuglodon. And as the aforesaid bony tubercles of 

 the porpoises are always found on or near the back-fin, it 

 has been assumed that in Zeuglodon the entire dorsal fin, 



