366 NOTICES OF THE MEETINGS [Web. 10, 
primary tissues of a tooth were briefly defined: they differ in 
hardness, the cement being least dense, the enamel most. 
The tubular structure of the dentine relates to the disposition 
of the hard material so as best to resist pressure, and to the cir- 
culation of plasma, transuded from the pulp through the dentine, 
so as to maintain a certain, though languid, vitality of the 
tissue. 
Some secondary modifications of the chief tissue of teeth were 
noticed under the names of osteo-dentine, vaso-dentine, vitro- 
dentine, dendro-dentine and labyrintho-dentine: the latter highly 
complex and beautiful modification, being due rather to a modi- 
fication of disposition, than of composition of the dentine itself. 
The singular labyrinthic interblending of the dentine and cement, 
reaches its maximum of complexity in the teeth of some gigantic 
extinct batrachian Reptiles, from the ‘Triassic formations, called 
from their distinct peculiarities “‘ Labyrinthodonts,”’ 
The chief varieties in the form of the teeth in Fishes were then 
enumerated, and more especially illustrated in the predatory Pike, 
the vegetarian Carp, the shell-crushing Myliobates and the coral- 
browzing Scarus. The elastic attachment of the teeth of the 
Lophius, and the mode of growth and succession of the Shark’s 
numerous teeth were explained. 
From the class of Reptiles examples of dental structure were 
selected; from the Serpent-tribes, in relation to the poison-appa- 
ratus, and from the Crocodile, in respect of the constant succes- 
sion and displacement of the teeth. The structure of the teeth 
of the extinct Iguanodon and Megalosaurus was also noticed. 
The Mammalian class might be divided in regard to the suc- 
cession of the teeth, into two groups—the Monophyodonts, or 
those that generate but one set of teeth, and the Diphyodonts, or 
those that generate two sets of teeth. 
The Monophyodonts include the Cetacea and the Bruta (Eden- 
tata of Cuvier); all the other Orders are Diphyodonts. 
The teeth of the Mammalia, especially the Diphyodonts, have 
usually so much more definite and complex a form than those of 
fishes and reptiles, that three parts are recognised in them: viz. 
the “ fang,” the ‘“ neck,” andthe “ crown.” The fang or root 
(radix) is the inserted part; the crown -(corona) the exposed 
part; and the constriction which divides these is called the neck 
(cervix). The term “ fang” is properly given only to the im- 
planted part of a tooth of restricted growth, which fang gradually 
tapers to its extremity; those teeth which grow uninterruptedly 
have net their exposed part separated by a neck from their im- 
planted part, and this generally maintains to its extremity the same 
shape and size’as the exposed crown. 
It is peculiar to the class Mammalia to have teeth implanted in 
sockets by two or more fangs; but this can only happen to teeth 
of limited growth, and generally characterises the molars and 
