xxix. 9 SEA-COWS 721 



large air-sacs. Sea-cows remain submerged only for relatively short 

 periods (10 minutes). The blood system shows retia mirabilia in the 

 brain and elsewhere, as in other aquatic mammals (p. 692). The brain 

 is small and the ventricles exceptionally large. The forebrain is 

 rounded but the rhinencephalon less reduced than might be expected 

 by comparison with whales. The neopallium is smaller and less folded 

 than in almost any other mammal of comparable size. The eyes are 

 small and protected by muscular lids; the animals do not see well. 

 The external auditory meatus is reduced to a channel a few milli- 

 metres wide, as in whales. Little is known of the hearing but reports 

 are that it is acute. 



In the manatees, the upper lip is greatly developed to form a strong 

 yet sensitive pad, used for cropping. The front parts of the jaws carry 

 horny pads for chewing. The teeth form a series of pegs, with two 

 transverse ridges; there may be up to twenty of them and those in 

 front drop out when worn. It has been supposed that there is a con- 

 tinual replacement from behind, as in elephants, but this is doubtful. 

 In the dugong the teeth are much reduced and the lower jaw carries 

 a horny pad; the upper carries a pair of tusks in the male and the pre- 

 maxillae are very large. The stomach is complex but not like that of 

 either the whales or other ungulates. The intestine is very long. 



The reproductive system shows such primitive features as ab- 

 dominal testes (with no signs that there was a descent in the ancestors) 

 and a bicornuate uterus. The placenta shows a zonary arrangement 

 and haemochorial structure, resembling that of elephants and conies. 

 The young are born in the water and nursed at pectoral teats, which 

 habit, with other features, may have produced some of the legends 

 of mermaids. 



Eocene fossils are known (*Protosiren) which, while definitely 

 sirenians, show distinct similarity to the ungulates of those times. 

 The nostrils were directed dorsally as in modern forms, but the tooth 

 row was complete and a small hind-limb was present. 



