THE CETACEA. 391 



part of the thoracic cavity to extend posteriorly over 

 almost the whole of the abdomen. The greatly elongated 

 lungs fiU this part of the thoracic chamber, while the broad 

 heart lies in its anterior and sternal portion. 



The arytenoid cartilages are not prolonged as in the 

 Cetacea. A broad and high epiglottis is capable of covering 

 the glottis completely. 



There is no third bronchus. 



The ciitaneoiis miiscle is largely inserted into the hn- 

 meiais, and the sub-caudal muscles extend forward as far 

 as the posterior lumbar vertebrae. The chief muscles of the 

 antibrachium and manus are j)resent. 



The male Sirenia possess vesiculse seminales. The uterus 

 is two-horned. 



There are two living genera of Sirenia — the Dugong 

 (Halicore), which is found upon the shores of the Indian 

 Ocean and of Australia ; and the Manatee {Manatus), which 

 is confined to the South American and African borders of 

 the Atlantic. 



A third genus, Rhytina, which had a coi'iaceous integu- 

 ment almost devoid of hair, and possessed no teeth, 

 abounded in Behring's Straits less than a century ago. 

 It is now altogether extinct. 



The Miocene genus, Halitheriuvi, appears to have pos- 

 sessed distinct, thoiigh small, hind limbs. 



lY. The Cetacea. — In this order of Mammalia the form 

 of the body is still more fish-like than in the Sirenia. 

 There is no trace of a neck, the contour of the head 

 passing gradually into that of the body. A horizontally 

 flattened caudal fin is always present ; and, very generally, 

 the dorsal integument is produced into a median, laterally 

 compressed, dorsal fin. The body is encased in a thick 

 smooth integument, beneath which a very thick layer of 

 fat is deposited. Hairs are almost entirely absent in the 

 adiUt state. 



As in the recent Sirenia, the anterior limbs alone are 

 present. Externally they do not present any indication of 



