HALICORE. 369 



the periphery of the tooth. Beyond this part the tubes proceed 

 with the usual gentle parallel primary curvatures to the periphery 

 of the dentine. The cells of the dentine become more conspicuous, 

 their boundaries being more opake, in the outer part of the dentine : 

 and the analogy which this structure presents to that of the dentine 

 in Platanista and Delphinus is interesting in connexion with the 

 outward cetaceous form of the Dugong ; the dichotomous divisions and 

 the minute lateral branches of the dentinal tubuli are not less con- 

 spicuous in the Dugong than in the carnivorous Cetacea. The clear 

 dentinal cells present an imbricated arrangement when viewed in 

 vertical section, and have a diameter of -f^th of an inch. 



The calcigerous cells of the cement closely resemble in their size, 

 form, and disposition those in the Platanista. 



The communications betw^een the tubes of the cement and 

 those of the dentine are clearly discernible in several parts of the 

 circumference of the latter substance, and the whole system of 

 tubes adapted to circulate the plasma of the blood through the 

 solid tissues of the tooth is, perhaps, in no animal better seen than 

 in the molar of the Dugong. The small portion of osseo-dentine 

 in the centre of the tooth is permeated by a few vascular canals, 

 which are derived from the remains of the pulp-cavity. 



The calcigerous tubes of the incisive tusks radiate from the 

 subcentral pulp-fissure with a well-marked double curvature, at first 

 convex, then concave, towards the extremity of the tusk. In a 

 transverse section, taken within an inch of the extremity of the 

 tusk, the dentinal tubes are grouped in fasciculi where they are so 

 crowded as to intercept the light, with narrow intervals where the 

 tubuli are fewer, but the unusual density of the dentine depends on 

 the clear and compact earthly salts combined with the animal matter 

 in the dentinal cells. 



In the female Dugong the whole of the extremity of the tusk 

 is surrounded by a thin coat of true enamel, which is covered by a 

 thinner stratum of cement. 



In the male's tusk the enamel (PL 95, fig. I , e) , though it may origi- 

 nally have capped the extremity, as in the female's, yet, in the body of 

 the tusk, it is laid only upon the anterior convex and on the lateral 



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