Digestive Si/ste?n General. 61 



distinct lamina), eacli of whicli is at the same time separated 

 and held together by the softer Cement which external to 

 the Enamel follows its every fold. Replacement. In almost 

 every species the teeth are formed in Alveoli ; but in some, 

 the tooth when completed and worn down is not replaced ; 

 while in others, two distinct sets are developed which have 

 been termed Permanent, and Temporary (Deciduous, or Milk) 

 teeth respectively. The Permanent and Deciduous teeth are 

 both formed simultaneously side by side from independent 

 portions of the primitive dental groove ; but the one destined 

 for early functional activity proceeds rapidly in its develop- 

 ment, while the other makes little progress until the time 

 approaches when it is called upon to take the place, by verti- 

 cal displacement, of the more precocious locum tenens*. The 

 process of Replacement is never repeated more than once in any 

 Mammal. Those species which generate only a single set of 

 teeth are termed Monophjdontsf (fioyog, once ; ^vw, I generate ; 

 odovg, tooth) ; those which generate two sets are termed 

 Diphyodonts {Ziq, twice, &c.). Some species, however, seem to 

 occupy an intermediate position in this respect, in that the 

 successional process is confined to a single tooth on each side 

 of the jaw+. Number. A few genera and species are devoid 

 of teeth% The so called 'typicaV number of teeth in this Class 

 is forty-four||, but the number varies excessively ; from one 

 fully developed^, to one hundred and ninety**. The ^typical' 

 number of true Molars in the Placental Mammalia with two 

 sets of teeth {i.e., Diphyodonts) is three. The more purely 

 carnivorous the species and the more it feeds upon living prey, 

 the less numerous are the Molars. Situation. True teeth im- 

 planted in Sockets are confined to the Maxillary, Premaxillaryff, 



* FloM^er. 



t Cete, In-Enamellata, Monotremata. + Infra. Marsupialia. 



Myrmccopliaga, Manis. Echidna. || Cf. Insectivora. Artiodactyla. 

 If Cf. infra. Situaiion. ** Cf. In-Enamellata. f f or 7^e>--nuixillar}-. 



