Chapter Five 



an 



MOUTH 



JlLxcEPT for the teeth where already noted the only aquatic mammals 

 whose mouths are of noteworthy interest are Ormthorhynchus, the 

 Cetacea, Hippopotamus, the walrus, and the Sirenia. 



Ormthorhynchus, or the platypus, has a mouth that is unique among 

 Mammalia. Except for its extreme broadness it superfically resembles 

 the bill of a duck, being used for the same purpose, but whereas the lat- 

 ter is horny, the bill of the former, although firm, is covered with soft, 

 moist, and extraordinarily sensitive skin. As the bones of both the man- 

 dible and rostrum are bifurcated, some play is permitted and the ani- 

 mal, according to Burrell (1927), can pucker the mouth sufficiently to 

 form a central suction tube whereby small life of the stream bottoms 

 may be acquired with the minimum of mud. Besides the serrated, 

 horny part of the lower lips which develops after the early loss of the 

 teeth, there is a pair of longitudinal structures in either jaw which 

 Burrell designates secateuring ridges. Cheek pouches are also found, 

 which the same authority considers are used mainly for holding fine 

 gravel as an aid in masticating chitinous or other hard food. 



Never having seen a living sirenian my conclusions regarding their 

 feeding actions have been derived from the literature. They are in the 

 habit of using the fore limbs for pulling aquatic growth toward them, 

 the action then being, according to reports and illustrations, for the 

 herbage to be grasped and thrust into the mouth by a unique interaction 

 of the lips and a pair of fleshy folds, one upon either side, which may 

 be designated as the side lips. There is surely considerable difference 

 in this labial equipment between the manati and dugong, but the various 

 postures which may be assumed (see figure 8) so utterly and ridicu- 

 lously change the expression of the creatures that we can judge little 

 from the illustrations, for the posture shown by one animal may or may 

 not be one that the other can adopt. Certainly the labial musculature 

 of the sirenians is very remarkable, and of it Murie (1872) wrote: 

 "Were the trunk of an elephant cut short at the root, or better still, left 

 entire, but contracted to a minimum of its long diameter, and with the 



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