THE PR0B08CIDIAK8. 



597 



Period a fossil Sirenian {Halitherium) inhabited the shores 

 ■of western Europe. 



In the structure of the skull, their dentition and their her- 

 bivorous habits the Sirenians in a degree connect the Ceta- 

 ceans with the Ungulates, and elephants. 



Order 7. Proboscidia. — Only two representatives of this 

 group are now in existence, the Asiatic and African elephant, 

 a number of other forms having become extinct. The group 

 is well circumscribed, when we consider the living species, 

 but in the early (Eocene) Tertiary Period tiiere existed forms 

 which indicate that the Proboscidians and Ungulates had a 

 common origin. In 

 the elephants the up- 

 per incisors are enor- 

 mously developed, 

 while there are none 

 in the lower jaw. 

 There are no canine 

 teeth, while the few 

 molars are large, trans- 

 versely ridged. In the 

 elephants the ridges 

 are numerous, the 

 spaces between tliem 

 filled with cement. 

 The young mastodon 

 has cement on the up- 

 per surface of the 

 tooth ; the ridges af- 

 terwards become free 

 and covered with 

 enamel. A peculiari- 

 ty in the elephant's skull is its large size, the brain cavity 

 being very small in proportion to the bulk of the skull itself. 

 To give lightness to what would be otherwise an insupportable 

 weight, the cranial bones contain numerous large air-cells 

 (Pig. 520). Another remarkable feature,from which the group 

 takes its name, is the trunk or proboscis, a long, thick, fleshy, 

 flexible snout, growing from the front edge of the nasal 



Fig. 519.— Skull of yonng elephant ; 22, premax- 

 illary hone containing the root of the tusk, k; 15, 

 nasal bone ; 7, par-etal bone or temporal region ; 

 26, malar, zygomatic arch ; i, lower jaw ; c, upper 

 jaw ; m, molar tooth ; 21, maxilla ; 11, frontal ; g^ 

 squamosal.— After Owen. 



