310 MOSTLY MAMMALS 
In a fossil porpoise (Delphinopsis freyerc) 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, 
