370 HALICORE. 



surfaces, but not upon the posterior concave side of the tusk ; which 

 is thickly coated with cement (fig. 1, c). This side accordingly is 

 worn away obliquely when the tusk comes into use whilst the enamel 

 maintains a sharp chissel edge upon the anterior part of the pro- 

 truded end of the tusk. 



The dentinal cells are of a subcircular form, with an average 

 diameter of y^th of an inch; they seem to form an unbroken net- 

 work, and in sections taken parallel with the course of the calcigerous 

 tubes present an imbricated character, as in Plate 95, fig. 1, the free 

 convex border of each compartment or cell being turned towards the 

 periphery of the tooth, near to which the cells diminish in size and in- 

 crease in number. The interspaces or outlines of the cells (fig. 2, b) 

 are dark or clear, as the change of focus causes them to intercept 

 or transmit the rays of light reflected from the mirror of the micros- 

 cope. In a section taken transversely to the course of the tubes 

 and parallel with the plane of the layer of cells, the reticulated takes 

 the place of the imbricated character : the dark extremities of from 

 ten to fourteen calcigerous tubes whose diameter is j^ooo^h of an inch, 

 may be seen included in the area of a single dentinal cell, as in 

 Plate 95, fig. 2. 



The presence of abortive teeth concealed in the sockets of the 

 deflected part of the lower jaw of the Dugong, offers an interesting 

 analogy with the rudimental dentition of the upper jaw in the 

 Cachalot and of both jaws in the foetal Whales : the arrested growth 

 and concealment of the upper tusks in the female Dugong, and 

 the persistent pulp-cavity and projection of the corresponding tusks 

 in the male, are equally interesting repetitions of the phenomena 

 manifested on a larger scale in the singular dental system of the 

 Narwhal ; but the habitual abrasion to which the tusks of the male 

 Dugong are subject prevents their closer resemblance to the male 

 Narwhal's tusk in regard to length. 



The simple implantation of the molar teeth and their compo- 

 sition are paralleled in the teeth of the Cachalot : their difference 

 of form, and the more complex shape of the hindmost tooth are 

 repetitions of characters which were present in the dentition of the 

 extinct Zeuglodon. 



