CETACEANS. 355 



usually twenty-three teeth in each ramus of the lower jaw of a 

 full-sized female Cachalot. 



The first-formed extremity of the tooth in the young Cachalot 

 may be tipped with enamel, but as yet no authentic example has 

 come under my notice. In all that 1 have examined, the summits 

 of the crown have been more or less abraded, and the tooth has 

 consisted of a hollow cone of dentine coated by cement, and more 

 or less filled by the ossified pulp. Irregular masses of this fourth 

 substance have been found loose in the pulp-cavity of large teeth ; 

 one of those which I divided had no foreign substance as a nucleus. 

 The external cement is thickest at the junction of the crown and 

 base, which are not divided by a neck. The laminated appearance 

 of the dentine is very conspicuous in the surface of polished sections, 

 such as that exhibited in PI. 89, fig. 2 ; the cause of that appearance 

 will be described in the next section ; in this figure a is the cement, 

 h the dentine and c the osseo-dentine or irregularly ossified pulp. 



144. Structure. — The dentine of the teeth of the Carnivorous 

 Cetacea is chiefly remarkable, as Retzius first observed, for the 

 number of calcigerous cells in it, arranged generally in planes 

 parallel to the superficies of the tooth and giving rise, in vertical 

 sections, to the appearance of concentric layers of dentine ; it is 

 also characterised by the free communication of the dentinal tubes, 

 either immediately or by their branches with these cells, and through 

 their minute prolongations, with the radiated cells of the cement. 

 The early closure of the pulp-cavity in the teeth of the Cetacea 

 may relate to this free communication of the minute canals adapted 

 to circulate the liquor sanguinis through the dental tissues, and to 

 maintain their vital connection with the rest of the organism. The 

 Cetacea in which I have studied the microscopic structure of the 

 teeth are the Belphinus Tursio, the Platanista and the Physeter 

 macrocephalus. The dentinal tubes from the upper part of the 

 pulp-cavity, in the Belphinus, ascend and curve, even with slight 

 convergence, towards the apex, then begin to diverge, and lower 

 down the tooth they pass out at right angles to the surface : thus far 

 their course is nearly parallel with one another, but in the fang they 

 assume more considerable and irregular curves. In the Cachalot 



A A 2 



