006 ZOOLOGY 



SECT. 



position of tlie iiarial aperture, which is situated comparatively far 

 forwards ; the limbs are not known: tliere 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 Avell as in the 

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

 and Australia, are represented by an extinct family, the 

 Squalodontida: (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 Bhyliiia ('* 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 

 leufjth 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 Avith simple 

 molars through a long series of gradations to the mouodactyle 

 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 Dinotho-idon (Fig. 1235) (Miocene and 

 Pliocene of Europe and India), which possess a pair of downward 1}'- 

 directed tusks in the lower jaw. The genus Pyrothcriinn, 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 Ilceritherium from the 



