xii PHYLUM CHORDATA 55 



devoid of roots, growing throughout life from persistent 

 pulps. Enamel is present as a thick layer on the ante- 

 rior convex surface only, which accounts for the .bevelled- 

 off character of the distal end, the layer of enamel being 

 much harder than the rest, which therefore wears more 

 quickly away at the cutting extremity of the tooth. The 

 second pair of incisors of the upper jaw are small teeth which 

 are lodged just behind the larger pair. In the lower jaw are 

 two incisors, which 'correspond in shape with the anterior 

 pair of the upper jaw. The remaining teeth of the upper 

 jaw are lodged in the maxillae. Canines, present in most 

 mammals as a single tooth on each side, are here entirely 

 absent, and there is a considerable space, or diastema, as it 

 is termed, between the incisors and the teeth next in order, 

 the pre-molars. Of these there are three in the upper jaw 

 and two in the lower. They are long, curved teeth devoid 

 of fangs, the first smaller than the others. Behind the pre- 

 molars are the molars, three on each side both in the upper 

 and lower jaws. 



Opening into the cavity of the mouth are the ducts of 

 four pairs of salivary glands. On the floor of the mouth is 

 the muscular tongue, covered with a mucous membrane 

 which is beset with many papillae. The roof of the mouth 

 is formed by the palate. The anterior part, or hard palate, 

 is crossed by a series of transverse ridges of its mucous 

 membrane. The posterior part, or soft palate, ends behind 

 in a free pendulous flap (the uvula} in front of the opening 

 of the posterior nares. Behind the mouth or buccal cavity 

 proper is the pharynx. The pharynx is divided into two 

 parts, an upper or nasal division, and a lower or buccal 

 division, by the soft palate. Into the nasal division open in 

 front the two posterior nares and at the sides the openings 

 of the Eustachian tubes. The nasal division is continuous 



