266 ANATOMY OF VERTEBRATES. 



Armadillo, which has ninety-eight teeth ; and in the Cetaceous 

 order by the Cachalot, which has upwards of sixty teeth, though 

 most of them are confined to the lower jaw; by the common 

 porpoise, which has between eighty and ninety teeth ; by the 

 Gangetic dolphin, which has one hundred and twenty teeth ; and 

 by the true dolphins (Delphinus), which have from one hundred 

 to one hundred and ninety teeth, yielding the maximum number 

 in the class Mammalia, 



Where the teeth are in excessive number, as in the species 

 above cited, they are small, equal, or sub-equal, and of a simple 

 conical form ; pointed, and slightly recurved in the common 

 dolphin ; with a broad and flattened base in the Gangetic dolphin ; 

 with the crown compressed and expanded in the porpoise ; com- 

 pressed, but truncate, and equal with the fang, in Priodon. The 

 compressed triangular teeth become coarsely notched or dentated 

 at the hinder part of the series in the great extinct cetaceous 

 Zeuglodon. The simple dentition of the smaller Armadillos, of 

 the Orycterope, and of the three-toed Sloth, presents a difference 

 in the size, but little variety in the shape of the teeth, which are 

 subcylindrical with broad triturating surfaces ; in the two-toed 

 Sloth, the two anterior teeth of the upper jaw are longer and 

 larger than the rest, and adapted for piercing and tearing, 

 fig. 215. 



Teeth are fixed, as a general rule, in all Vertebrates. In 

 Mammals the movements of the teeth depend on those of the 

 jaw-bones supporting them, but appear to be independent in the 

 ratio of the size of the tooth to the bone to which it is attached : 

 the seemingly individual movements of divarication and approxi- 

 mation observable in the large lower incisors of the Batliyergus 

 and Macropus, 1 are due entirely to the yielding nature of the 

 symphysis uniting the two rami of the lower jaw, in which those 

 incisors are deeply and firmly implanted. 



In Man, where the premaxillaries early coalesce with the 

 maxillary bones, where the jaws are very short, and the crowns of 

 the teeth are of equal length, there is no interspace or ' diastema ' 

 in the dental series of either jaw, and the teeth derive some 

 additional fixity by their close apposition and mutual pressure. 

 No inferior Mammal now presents this character; but its im- 

 portance, as associated with the peculiar attributes of the human 

 organisation, has been somewhat diminished by the discovery of 

 a like contiguous arrangement of the teeth in the jaws of a few 

 extinct quadrupeds ; e. g., A?ioplothcrium, Nesodon, and Dichodon. 2 



1 xxv. vol. i. p. 285. '-' clxxx. fig. 130. 



