PLATANISTA. 



437 



pulp cavity is still open are laterally compressed, the longitudinal being about twice 

 as long as the transverse diameter of the orifice ; but when the cavity is closed, the 

 base of the tooth longitudinally is so much laterally compressed as to assume the 

 character of an osseous plate, carrying obliquely on its apex the portion of the tooth 

 which originally appeared beyond the gum, and retaining all its youthful characters 

 and with an unworn tip, but with its anterior border in the case of the lower teeth 

 deeply grooved by the friction of the upper tooth apposed to it, the latter teeth 

 becoming similarly worn on their posterior borders/ Associated with this closure 

 of the pulp cavity occurs an increase in the dimensions of the base of the tooth. 



Fig. 13. 



123 4 5 6 7 8 



Series of teeth of Platanista gangetica, to illustrate their growth and wear age. All drawn of natural dimensions. 

 1 — One of the long sharp front teeth of a young animal. 



2 — A tooth from the posterior end of the row of lower jaw of a young animal. 

 3 — An adult tooth where wearage of crown is most irregular, and only a small piece of the enamel remains : pillars 



of cementum are produced at the fang. 

 4— Shows an adult tooth where great widening and flattening of the fang has taken place, the crown heing still covered 



with enamel, and long and conical in shape. 

 5 — Another adult tooth equally showing increased flattened root growth, but diminishing crown ; a cap of enamel still 



exists, but the dentinal substance immediately below is highly polished on the outside, the pearly lustre simulating 



enamel. 

 6— An older more worn tooth, where the crown rubbed down to a low nipple shape is destitute of enamel, and even the 



dentine is altered into the so-called osteodentine. 

 7 — A tooth now almost crownless, and the body a mere strip uniting the columnar superadded root of cementum. 

 8 — A further stage where even absorption of the false root has proceeded, while the elements of the original tooth itself 



are not only rubbed down, but utterly gone : what remains is pure bony substance, substituted, but functionally 



answering for a dental organ. 



In a more advanced stage the jaws are much deepened, and osseous deposits are 

 observed to have taken place in the sockets, the jaw at the same time growing in 

 all its dimensions, the bases of the teeth having noticeably increased in size, while 

 the free original portion has become worn away on its sides and reduced in length 

 by wear and breakage. At this stage, the teeth are not so deeply set in their sockets, 

 a circumstance due to the fact that they seem to be outwardly pushed by the accumu- 

 lation of osseous growths in the sockets themselves, and by their own enlargement. 



With increasing age the portions of the teeth that originally pierced the gum 

 are worn away entirely, until nothing but their enlarged bases remain, the free ends 



1 Odont. p. 353. 



