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 t} r pe, 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, 

 Avith 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, 1 

 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 Avith the primitive forms of many other 

 types. The forward position of the narial aperture and the length 

 and flatness of the nasal bones, Avhich distinguish them from all 

 existing forms, Ave must also suppose to be a character at one time 

 common to all Cetaceans, though noAV retained (but to a less degree) 

 only by the Mystacocetes. Even Squalodon, Avhich 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 Avith the ordinary Dolphins in its essential cranial 

 characters. 



The origin of the Cetacea is at present invoked in much ob- 

 scurity. They present no signs of closer affinity to any of the 

 loAver classes of A r ertebrates than do many other members of their 

 own class. Indeed in all that essentially distinguishes a mammal 

 from the oviparous Arertebrates, Avhether 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 AAdiich the heterodont type of dentition was fully 



1 The ankylosed mass of cervical vertebrae, on which the genus Palceocetus 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. 



