344 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



In the dolphins, where the olfactory sense is greatly 

 reduced or perhaps altogether lost, no olfactory fossa 

 is to be seen in a longitudinal section of the cranium ; 

 in correlation with their habits, and the large size of 

 the mouth cavity, and the length of the premaxillse 

 and maxillae which form its roof, the nasals are 

 reduced in size and placed far up on the surface of the 

 skull ; the anterior nares consequently look upwards 

 instead of forwards, and are therefore the first to 

 appear above the surface of the water, when these 

 pulmonate aquatic animals come up to breathe air. 

 Just as in crocodiles, whose habits are so far essentially 

 similar, the ptery golds develop transverse plates, 

 the formation of which throws the posterior nares 

 very far back on the roof of the mouth, and thereby 

 approximates them to the opening into the trachea. 



It is important to observe that quite a different 

 set of arrangements obtains in the Sirenia, which, 

 instead of being active or marine forms, are sluggish, 

 and live much at the bottom of shallow rivers ; here 

 the bones are exceedingly dense, the anterior nares 

 look forwards, but are of very large size, and no special 

 plates of the pterygoid prolong the nasal passages 

 backwards. When we compare the structural adap- 

 tations of the Sirenia and the Cetacea, it is impossible 

 to avoid seeing, on purely theoretical grounds, which 

 of the two has the advantage ; and this consideration 

 may be supported by the abundance of the species 

 and specimens of the latter as compared with the 

 former. 



The ant-eater is another form in which the 

 posterior nares are placed very far back owing to the 

 development of pterygoidal plates to form a floor for 

 the air passages ; in this case an explanation is possibly 

 to be found in the fact that such a disposition, by 

 means of which the nares and the glottis are brought 

 into much closer proximity, would be an advantage in 



