DENTAL TISSUES. 



363 



The examples arc extremely few, and peculiar to the class 

 Pisces, of calcified teeth wliich consist of a sing-lc tissue, and this is 

 always a modification of dentine. Tlie large pharyngeal teeth oF 

 the AVrassc (Lahrus) consist of 

 a very hard kind of unvascular 



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dentine. Fig. 240 shows a ver- 

 tical section of one of these teeth, 

 supported upon the vascular 

 osseous tissue of the pharyngeal 

 bone : p is the pidp cavity. 



The next stage of complexity 

 is where a portion of the dentine 

 is modified by vascular canals. 

 Teeth, thus composed of dentine 

 and vaso-dentine, are very com- 

 mon in fishes. The hai'd dentine 

 is always external, and holds the 

 place, and performs the office, of 

 enamel in the teeth of higher 

 animals ; but it is only analogous 

 to enamel, not the same tissue. 

 Fig. 241 exemplifies this struc- 

 ture in a longitudinal section of the tooth of a Shark (Lriiuna). 



The mtilars of the Dugong (^HaUcore) are coni])osed of dentine 

 and cement . the latter substance forming a thick outer layer, 

 fig. 242, c. 



In the teeth of the Cachalot (Phi/scter) the pulp-ca\ ity of the 

 growing tooth becomes filled up by osteodentine, the result of a 

 modified calcification of the dentinal pulp ; when the tooth presents 

 three tissues, as shown in fig. 239, in which c is the thick external 

 cement, d the hard dentine, and o the osteodentine ; sometimes 

 developed in loose stalactitie-shaped nodules. 



In the teeth of the Sloth, and its great extinct congener, the 

 Megatherium, the hard dentine is reduced to a thin layer, fig. 2.38, 

 t, and the chief bulk of the tooth is made up of a central body of 

 vaso-dentine, ib. v.. and a thick external crust of cement, ib. c. 



Besides the number of constituent tissues teeth become 'complex' 

 in structure by the proportion and disposition, chiefly inflection, 

 of more or fewer of those tissues. 



Certain fishes and the extinct ' Labyrinthodont ' reptiles ex- 

 hibit this complexity in a remarkable degree. In fig. 243, the 

 tooth of tlic Lahj/rinthodon salainandro'idcs feel)ly indicates its 

 singular structure by the longitudinal striie. But every streak is 



