HOROLOGIES OF TEETH. 367 



A vascular matrix buds out in the shark from the membrane 

 covering the jaw, as in the deer from that covering the cranium, 

 and the blood-vessels, ramifying through such matrix, convev the 



*/ C3 f-J / 



phosphate of lime which hardens it ; each ultimate ramification 

 that radiates a system of dentinal tubes in the shark's tooth, cor- 

 responds with the same ramification of the artery radiating lacunal 

 tubes in the matrix of the deer's antler. 



After the tooth of the shark has been worn by the uses for 

 which it was calcified, it is shed like the antler, and is succeeded 

 by another. There is merely a difference in the place of suc- 

 cession, the new tooth rising close to, but not, as in the antler, 

 directly under, the base of the old. 



But the basis from which the matrix of both tooth and antler 

 grows is homologically the same. In both instances the gum, or 

 corium, is pushed out by the growing matrix : in the deer it forms 

 the ' velvet ' which peels away from the ossified matrix, in the shark 

 it is hardened into the enamel-like layer covering the matrix. 



These are the differences that can be predicated in reference to 

 the histological homology of the parts in question, and the shark's 

 tooth answers to the deer's antler, plus the outer enamel-like 

 .covering, in mode of development, structure, growth, shedding, and 

 succession. They correspond, alike, with osseous texture ; and, 

 under a less genus, with the parts of the dermo-skeleton. 



But the tooth of a shark is homologous with that of a porpoise; 

 therefore, teeth are referable to the dermo- or entero-skeletal 

 parts of the osseous system. 



Descending to the special homologies, we find that the idea 

 of a recognition of answerable teeth in different animals has 



o 



prevailed, more or less vaguely, in Anatomy, from an early period 

 of the science. 



When ' incisors,' f canines,' and ' molars ' were predicated of 

 the dentition in different species, homologous teeth were re- 

 cognised so far as the characters of those classes of teeth were 

 defined and understood. 



The Cuviers l went a step further, and distinguished the molar 

 teeth into ' false' and ( true,' into 'carnassial' and ( tubercular.' De 

 Blainville pointed out a particular tooth by the name of f principal,' 

 which he believed himself able to trace from species to species. 2 



The first step in this inquiry is the elimination of those classes 

 of Vertebrata and orders of Mammalia in which homology cannot 

 be predicated of individual teeth. This limits the work to the 

 group of mammals here termed i Diphyodonts. 5 



1 cxx". and cxxi". a The line' Blainville' runs through that tooth in fig. 293. 



