MAMMALIA. 



545 



anterior nares at the front end of the facial region. The 

 nasal chambers serve the dual functions of smell and of 

 respiration. The length of the nasal chambers and the 

 distance between the anterior and posterior nares, combined 

 with the great exposed surface of the turbinals, ensure the 

 activity of the olfactory sense. 



In the Cetacea the serial olfactory sense is of little or no 

 use, whilst a rapid and easy passage of air to the lungs is 

 essential. Hence the anterior nares have progressed back- 

 wards till they come to lie vertically over the internal nares, 

 and the nasal " chamber '' of terrestrial types, with its 

 complex turbinals, has been converted into a simple pair of 

 short passages, with no turbinals, leading directly downwards 

 to the glottis. In terrestrial types the roof of the nasal 

 chambel: is formed by the nasals and partly the frontals. 

 Here the nasals and frontals are pushed backwards before 

 the retiring nostrils. The frontals squeeze the parietals to 

 the sides and meet the supraoccipital, whilst the nasals are 

 pressed against the front wall of the cranial cavity. Hence 

 the " rostrum " represents only the ventral or alimentary 

 part of the mammalian facial region, consisting solely of 

 the premaxillse — which follow the nostrils backwards and 

 become very elongated — the maxillae, mesethmoid and the 



(From Flower and Lyddeker.) 



The maxillae, premaxillae and mandibles bear a single 

 row of small teeth, very numerous and all of the same size 

 (homodont). Each tooth has a single root, and in the 

 porpoise is scoop-shaped 



and raised on a short Fig. 376.— Teeth of Porpoise x 2. 

 base. (In the dolphin 

 each is a simple conical 

 point. ) There are usual- 

 ly about twenty-five on 

 each side, upper and 

 lower jaws, and as they 

 are homodont we can 

 use no dental formula 

 but 14 (dolphin 



^^ to 

 ^^). There is no suc- 



cession (monophyodont), but there are said to be traces of a 



second or permanent dentition which is only transitory and 



M. z^ 



