. Elementary text-book of zoology. MAMMALIA. S4S 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 int
. Elementary text-book of zoology. MAMMALIA. S4S 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 chamber 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 maxillse, mesethmoid and the vomer. The maxillae, premaxillse 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 (From Flower and Lyddekee.) 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 If (dolphin ^ to fg-). There is no suc- cession (monophyodont), but there are said to be traces of a second or permanent dentition whi
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