xv GEOLOGICAL HISTORY 227 



the teeth of the Odontocetes, and with some characters more 

 like those of the generalised mammalian type than of any of 

 the existing forms. In fact Zeuglodon is precisely what we 

 might have expected a priori an ancestral form of whale to 

 have been. The remarkable smallness of its cerebral cavity, 

 compared with the jaws and the rest of the skull, so different 

 from that of modern Cetaceans, is exactly paralleled in the 

 primitive types of other groups of mammals. The teeth are 

 markedly differentiated in different parts of the series. In 

 the anterior part of both jaws they are simple, conical, or 

 slightly compressed and sharp pointed. The first three of the 

 upper jaw are distinctly implanted in the premaxillary bone, 

 and so may be reckoned as incisors. The tooth which succeeds, 

 or the canine, is also simple and conical, but it does not greatly 

 exceed the others in size. This is followed by five teeth with 

 two distinct roots and compressed pointed crowns, with 

 denticulated cutting edges. It has been thought that there 

 was evidence of a vertical succession of the molar teeth, as in 

 diphyodont mammals, but the proof of this is not quite satis- 

 factory. Unfortunately the structure of the limbs is most 

 imperfectly known. A mutilated humerus has given rise to 

 many conjectures ; to some anatomists it appears to indicate 

 freedom of motion at the elbow -joint, while to others its 

 characters seem to be those of the ordinary Cetacea. Of the 

 structure of the pelvis and hind -limb we are at present in 

 ignorance. ( 



From the middle Miocene period fossil Cetacea are abundant, 

 and distinctly divided into the two groups now existing. The 

 Mystacocetes, or whalebone whales, of the Miocene seas were, 

 as far as we know now, only Balcenopterce, some of which (as 

 the genus Cetotherium) were, in the elongated flattened form 

 of the nasal bones, the greater distance between the occipital 

 and frontal bones at the top of the head, and the greater 

 length of the cervical vertebrse, more generalised than any 

 now existing. In the shape of the mandible also, Van 

 Beneden, to whose researches we are chiefly indebted for a 

 knowledge of these forms, discerns some approximation to the 

 Odontocetes. Eight whales {Balcena) have not been found 

 earlier than the Pliocene period, and it is interesting to note 



