MICROSCOPIC STRUCTURE. 511 



worn upon their outer side. Each molar has a short, thick, simple 

 and solid root. 



The canines are developed only in the upper jaw, but are of 

 enormous size, descending and projecting from the mouth, like 

 tusks, slightly incUned outwards and bent backwards ; they present 

 an oval transverse section, with a shallow longitudinal groove along 

 the inner side, and one or two narrower longitudinal impressions 

 upon the outer side ; the base of the canine is widely open, its growth 

 being uninterrupted. 



The food of the Walrus consists of sea-weed and bivalves : the 

 molars are well adapted to break and crush shells ; and fragments of a 

 species of Mya have been found, with pounded sea-weed, in the 

 stomach. The canine tusks serve as weapons of offence and defence, 

 and to aid the animal in mounting and clambering over blocks 

 of ice. 



189. Composition and microscopic structure. — The teeth of the Car- 



nivora, with the exception of the aberrant amphibious forms in which 



we have been last considering the outward modifications of the dental 



system, so closely correspond in their intimate structure, both with 



each other and with those of the Quadrumana, as to require here only 



a brief and general notice. They all enter into the category of 



' simple teeth ;' that is, the dentine or main body is not penetrated 



by folds of the other component tissues, but has an even exterior, 



covered at the part forming the crown with enamel, and having a 



general outer investment of cement, the coronal layer forming too 



thin a film to manifest any of the radiated cells. The dentine is of 



the kind called * hard or unvascular' : the tubuli are rather finer than 



in the human teeth ; they have the same general direction from the 



pulp-cavity, but present stronger primary curvatures, more frequent 



dichotomous divisions, and more numerous minute lateral branches, 



which latter usually curve from the trunks at right angles. The 



dentinal cells are subhexagonal, about g^th of an inch in diameter, 



with the peripheral contour forming almost a regular curve. In the 



Seals the dentine forms usually a smaller proportion of the tooth 



than in the terrestrial Carnivora ; the characteristic thickness of 



the roots in this family is principally due to the thick covering of 



