THE CETACEA. 333 



tensive arterial and venous retia mirabilia are developed in 

 Manatus. In consequence of the great length of the thoracic 

 region and the brevity of the sternum, the diaphragm takes a 

 very unusual course, extending very obliquely from before 

 backward, and causing the upper part of the thoracic cavity to 

 extend posteriorly over almost the whole of the abdomen. 

 The greatly-elongated lungs fill this part of the thoracic cham- 

 ber, while the broad heart lies in its anterior and sternal portion. 



The arytenoid cartilages are not prolonged as in the Ceta- 

 cea. A broad and high epiglottis is capable of covering the 

 glottis completely. 



There is no third bronchus. 



The cutaneous muscle is largely inserted into the humerus, 

 and the sub-caudal muscles extend forward as far as the pos- 

 terior lumbar vertebrae. The chief muscles of the antibrachium 

 and manus are present. 



The male Sirenia possess vesiculsB seminales. The uterus 

 is two-horned. 



There are two living genera of Sirenia — the Dugong 

 (JTftlicore), 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, Mhytina, which had a coriaceous integu- 

 ment almost devoid of hair, and possessed no teeth, abounded 

 in Behring's Straits less than a century ago. It is now alto- 

 gether extinct. 



The Miocene genus, Salitherium, appears to have pos- 

 sessed distinct, though small, hind-limbs. 



IV. 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 gradu- 

 ally into that of the body. A horizontally-flattened caudal 

 fin is always present ; and, very generally, the dorsal integu- 

 ment is produced into a median, laterally-compressed dorsal 

 fin. The body is incased in a thick smooth integument, be- 

 neath which a very thick layer of fat is deposited. Hairs are 

 almost entirely absent in the adult state. 



As in the recent Sirenia, the anterior limbs alone are 

 present. Externally they do not present any indication of 

 division into brachium, antibrachium, and manus, but have the 

 form of a broad, flattened paddle, without any vestiges of nails. 



The one or two apertures by which the cavity of the iioae 



