232 ; CETACEA 
mammalian structure. It is therefore interesting to find an 
apparently allied form well represented among the earliest fossil 
remains of Cetaceans in Europe. Almost all the other members of 
the suborder range themselves under the two principal heads of 
Ziphioids (or Physeteroids) and Delphinoids. The former is an 
ancient and once abounding type, of which the Sperm Whale 
(Physeter) is a highly specialised form. Among the latter, Globi- 
cephalus is a modified form as regards the structure of its anterior 
extremity, and Monodon as regards its dentition, while Delphinus, 
with the various allied genera, may be regarded as the domi- 
nating type of Cetaceans at the present day, abundant in slightly 
differentiated species and also in individuals. They are in this 
respect to the rest of the order much as the hollow-horned 
Ruminants are to the other Ungulates. 
The earliest Cetaceans of whose organisation we have anything 
like complete evidence are the Zeuglodonts of the Eocene period,! 
which approach in the structure of the skull and teeth to a much more 
generalised mammalian type than either of the existing suborders. 
The smallness of the cerebral cavity compared with the jaws and the 
rest of the skull they share with the primitive forms of many other 
types. The forward position of the narial aperture and the length 
and flatness of the nasal bones, which distinguish them from all 
existing forms, we must also suppose to be a character at one time 
common to all Cetaceans, though now retained (but to a less degree) 
only by the Mystacocetes. Even Sgualodon, which in its heterodont 
dentition so much resembles Zeuglodon as to have been placed by 
some zoologists in the same genus, entirely differs from it, and 
conforms with the ordinary Dolphins in its essential cranial 
characters. 
The origin of the Cetacea is at: present involved in much ob- 
scurity. They present no signs of closer affinity to any of the 
lower classes of vertebrates than do many other members of their 
own class. Indeed in all that essentially distinguishes a mammal 
from the oviparous vertebrates, whether in the osseous, nervous, 
reproductive, or any other system, they are as truly mammalian as 
any other group. Any supposed marks of inferiority, as absence 
of limb structure, of hairy covering, of lachrymal apparatus, etc., are 
obviously modifications (or degradations, as they may be termed) 
in adaptation to their special mode of life. The characters of the 
teeth of Zeuglodon and other extinct forms, and also of the foetal 
Mystacocetes, clearly indicate that they have been derived from 
mammals in which the heterodont type of dentition was fully 
1 The ankylosed mass of cervical vertebr, on which the genus Paleocetus was 
established, was regarded by its describer as having probably come from the 
Kimeridge Clay, but the mineral condition of the specimen points to the Red 
Crag as the place of origin. 
