PLATANISTA. 439 



tissues of the tooth are vastly changed. The enamel is limited to a thin tiny 

 cap, and the dentine, with its central pulp cavity, is reduced to ahout a third or even 

 less of its original proportions, according to the varying circumstances of age, 

 position of tooth, &c. The mass of the dental suhstance has now become osteo- 

 dentine or cemental in nature. True hone lacunge and canaliculi freely intermingle 

 with what was before dental tubuli, and at the fang and the base of the body even 

 quite supplant dentine. 



In another tooth possibly older, but at all events one taken from the lower 

 jaw behind its middle (PL XXXVI, fig. 7), apical enamel was barely perceptible, 

 and the dentinal substance was still further reduced in dimension. 



In yet another tooth from the posterior tlm^d of the lower jaw (fig. 8) the 

 substitution of tissues had gone still further, no enamel being left, and pure dentinal 

 structure was limited to little more than the apex. In fact, the tooth, save a 

 very small portion, was converted into osteodentine. 



In quite old worn-down teeth bony tissue alone remained, or only with diffi- 

 culty could traces of dentine tubuli be recognised. 



It is evident, then, that as the bases of the teeth in this genus increase in size,^ 

 it is due to substitution of true osseous tissue for dentine, and that, associated with 

 this, there is a gradual outward forcing of the teeth which leads to the complete dis- 

 appearance in the adult of the elements of the tooth originally exposed above 

 the gum, the place of which is taken by a tooth of bone, the product of a growing 

 osseous base, and many times larger than the original tooth which was composed of 

 all the structures that enter into the formation of Mammalian teeth generally. 



The bases of the teeth of Glohicephalus deductor appear also to undergo a 

 considerable change in size from youth to age, and probably other Cetaceans will be 

 found to illustrate similar changes. 



Tharynx. — In the half- grown individual, the pharynx is defined behind the 

 epiglottis by the presence of a more or less continuous transverse fold, the folds 

 behind it being longitudinal ; but no such separation between oesophagus and pharynx 

 occurs in the adult, in which it is thrown into longitudinal folds continuous with 

 those of the fauces, but much feebler, and prolonged into the oesophagus. In the 

 semi-adult animal the pharynx has a transverse capacity of 2" 50 inches, while in an 

 adult it measures 5 inches in the same direction. The openings of racemose glands 

 are numerous on the floor of the cavity and at the base of the epiglottis, but they 

 are all but absent on its sides and roof. 



The tube of the larynx has been twisted to the left side in all the individuals I 

 have examined, and been firmly grasped by the sphincter of the posterior nares into 

 which the tube is generally found projecting. In the foetus it is also found occupying 

 a similar position, and has the same sinistral asymmetry which is extended to the 

 tongue in the adult as well as in the young ; this organ being more or less tilted up to 

 the left side, so that the straight course to the oesophagus passes the right side of the 

 epiglottis, which is doubtless the course followed by the food in the act of swallowing. 



• Owen, Odont., p. 352. 



