VII.] 



THE EXTERNAL SKELETON. 



257 



may meet with alveoli thoroughly complete, like those of 

 man, as in most members of his class. 



These complete alveoli, however, may be in some respects 

 different from those of man. They are so where teeth grow 



FIG. 224. INNER SIDE OF LOWER JAW OF PLEURODONT LIZARD, 

 showing the teeth attached to the inner surface of its side. 



from permanent pulps throughout the whole of life like the 

 tusks of the Elephant and the teeth of many other beasts 

 (e.g. the cutting teeth of the Squirrel). In such cases, of 

 course, the deeper part of the alveolus is quite wide instead 

 of being attenuated as in us. 



23. The NUMBER of teeth in man is interesting as being 

 not very far from that which is typical of the great bulk of 

 the class to which he belongs. It is identical with that exist- 

 ing in the whole of the Apes which inhabit the old world, and 

 those of the new world only differ from him by the presence 

 of one more pre-molar or by the absence of a molar on each 

 side of each jaw. 



In man's own class the number of teeth developed may be 

 very great, as in the Dolphins, where the greatest number 

 is reached in Pontoporia, namely 220. 



The large Armadillo (Priodon] may have as many as 

 ninety. They may be reduced to two, as in the Narwhal. 



If we pass out of man's class we may find teeth many or 

 few, as in Reptiles ; but amongst Fishes we meet with every 

 extreme, from a single pointed tooth on the roof of the 

 mouth, as in Myxine, or two above and two below (flat and 

 crushing), as in Ceratodus, up to such a multitude that to 

 count them would be a task both useless and difficult, as in 

 Murcena, the Pike, or Osteoglossum. 



24. The FORMS presented by the" teeth of man agree more 

 or less closely with what we find in many members of his 

 class, but have no relation, or only the most general and 

 distant one, to the teeth of cold-blooded Vertebrates. 



Although the shape of the teeth is nearly the same in all 

 men, nevertheless the wisdom teeth of those races which are 



S 



