606 ZOOLOGY SECT. 



position of the narial aperture, which is situated comparatively far 

 forwards ; the limbs are not known : there were irregular dermal 

 bony plates. The remains of both Whale-bone Whales and 

 Toothed Whales occur abundantly in Pliocene deposits, some 

 belonging to extinct, others to existing, genera. Toothed Whales 

 occur also in Miocene formations, and there, as well as in the 

 Pliocene and Pleistocene of Europe, North America, New Zealand, 

 and Australia, are represented by an extinct family, the 

 Squalodtmtidce (Fig. 1234), with heterodont dentition. 



The order Sirenia is first met with in the Eocene, and was repre- 

 sented in that and succeeding periods by several extinct genera, of 

 which Halitherium is the best known. These were characterised 

 by the possession of upper incisors, in some cases of canines, of 

 enamelled pre-molars and molars, of a milk-dentition, and of small 

 vestiges of femora. The family of the Dugongs is represented by 

 a form nearly allied to the existing genus in the Pliocene of 

 France, and probably by another genus in the Tertiary of California. 

 The family of the Manatees is not known to be represented by any 

 fossil forms. The recently extinct Rhylina (" Steller's Sea-Cow "), 

 which lived within historic times in the Behring's Straits, was 

 the largest known member of the order, and sometimes attained a 

 length of seven or eight metres. 



The Tertiary Ungulata comprise an immense number of 

 forms, including numerous extinct families, into an account 

 of which it would be going beyond the scope of the present work 

 to enter. In the Artiodactyle series there is to be traced a pro- 

 gressive union and coalescence of the third and fourth metacarpals 

 to form the cannon-bone, a progressive reduction of the lateral 

 digits, and a progressive development of horns or of tusks absent 

 or rudimentary in the earlier representatives of the sub-order. In 

 the Perissodactyle series the reduction of the lateral toes reaches 

 its maximum in the existing genus Equus. 'The history of this 

 reduction, together with the development of other characteristic 

 features, can be traced from pentadactyle forms with simple 

 molars through a long series of gradations to the monodactyle 

 Horses with their complexly folded molars. Similar genealogies, 

 though not always so complete, can be traced for the Tapirs and 

 Rhinoceroses, and for the Deer, Camels, and Pigs. 



The order Proboscidea was represented in Tertiary and Pleisto- 

 cene times, not only by forms allied to those now living though 

 sometimes, as in the Mammoths, of much greater size but also 

 by an extinct family, the Dinotheridce (Fig. 1235) (Miocene and 

 Pliocene of Europe and India), which possess a pair of downwardly- 

 directed tusks in the lower jaw. The genus Pyrotherium, from the 

 Patagonian Tertiary deposits, was a primitive Ungulate, with a 

 pair of short tusk-like incisors in the lower jaw, which may have 

 been related to the Proboscidea : and Maeritherium from the 



