310 ORDERS OF MAMMALIA. 



The Dugono's (^Hcdicorc, fig. 617, a) have or 



5 — 5 6 — t) 



molar teeth in tlie young condition, but tliese are never all 



in use at one time. The molars are without enamel, and 



are single-rooted. Inferior incisors are present in the young 



animal, but are wanting in the adult. Tlie upper jaw carries 



two permanent incisors, which are entirely concealed in the 



jaw in the females, but which increase in size in the males 



with the age of the animal, till they become pointed tusks. 



The Dugongs are very similar in appearance and habits to 



the Manatees, but they are more exclusively marine animals, 



and feed chiefly upon sea-weeds. 



The genus Bhytina, at one time abundant on the north- 

 west coast of North America, appears to have been com- 

 pletely exterminated about the middle of the eighteenth 

 century. In this curious type there were no true teeth, but 

 the place of the molars was taken by large lamelliform fibrous 

 structures, one on each side of each jaw. 



As regards their distrihution in time, the earliest known 

 remains of this order appear in the Eocene Tertiary of Egypt, 

 where we find the Eotherium Egyptiacum, apparently allied 

 to the living Manatees. Of the same age, perhaps, is the 

 interesting form described by Owen from the Tertiary of 

 Jamaica under the name of Frorastomus sircnoides. This type 

 is remarkable as possessing upper and lower canines in addi- 

 tion to incisor and molar teeth, the dental formula being — 



.3—3 1—1 5—5 3—3 ,„ 



3— 3(?)' 1—1' ^ 5—5 3—3 



The molars are enamelled, and the incisors are small ; the 

 genus thus appearing to be allied to the Manatees, though of 

 a more generalised type. 



In the Miocene period Sirenians appear to have been 

 comparatively abundant, though mostly referable to extinct 

 types. In deposits of this age in America, however, occur 

 remains which have been referred to the existing genus 

 Mancdus. The most important Miocene genus, extending 

 its range into the Pliocene, is Hcddlicrium (fig. 618), the 

 skeleton of which is now tolerably well known. In this 



