SPAROIDS. 97 



placed by the absorbent process, and their successors become in 

 their turn anchylosed to the bony plate which, in the progress of 

 growth, has at length reached the level of the alveolar margin. 



A vertical longitudinal section of the incisor of the Sargus,{l) 

 sufficiently thin to be examined microscopically by transmitted light, 

 shows that it consists of a central body of compact dentine, composed, 

 as in the human tooth, of fine, parallel, close-set calcigerous tubes ; 

 and that the crown is covered by a thick layer of a distinct substance 

 analogous to enamel. 



The structure of the external substance is, however, very unlike 

 that of the true enamel of the human or mammiferous tooth : other 

 broadly-marked distinctions are at once seen in the continuation of 

 the pulp-cavity to the apex of the crown, and in the expanded fang 

 or base of the tooth. 



The calcigerous tubes of the ivory, in the body of the tooth, make 

 a bend when they leave the pulp-cavity, with the convexity turned 

 towards the apex ; they are then slightly curved in the opposite direc- 

 tion, and again bend outwards, with the convexity as in the first curve, 

 thus describing a beautiful sigmoid undulation. At the base of the 

 tooth the direction of the calcigerous tubes is more transverse, and 

 the number of undulations is greater. Near the apex the middle bend 

 is gradually lost, and the tubes as they proceed outwards describe a 

 simple curve, with the convexity next the apex of the tooth. 

 The secondary curvatures of the calcigerous tubes are beautifully and 

 minutely undulatory (a, fig. 2). The main tubes at their commencement 

 near the base of the conical cavity of the pulp are more irregu- 

 larly flexuous, for a short distance than in the body of the tooth. They 

 send off minute branches throughout their whole course, at very acute 

 angles, except near their extremities, where the branches bend out- 

 wards and are consequently more readily discerned. The peripheral 

 extremities of the calcigerous tubes seem to terminate somewhat ab- 

 ruptly in a clear and apparently structureless space or stratum, which 

 intervenes between the tubuli of the ivory and those of the enamel. 

 External to this is an opake stratum, apparently composed of the ex- 



(l) PI. 43, fig. 2. 



H 



