218 A MANUAL OF DENTAL ANATOMY. 



which it, to borrow a phrase from geological science, has an 

 " outcrop." 



In Lamna the second and third rows of teeth are only 

 partially erect, the rows behind these lying recumbent, and 

 being in the fresh state covered in by the fold of mucous 

 membrane, which, being dried and shrunk in the specimen 

 figured, falls short of its original level. 



Thus rows of teeth originally developed at the base of 

 the jaw are carried upwards, come to occupy the foremost 

 position on the border of the jaw, and are cast off when 

 they pass the point/ in the figure. It is thus easy to under- 

 stand why sharks' teeth are so abundantly found in a fossil 

 condition, although other indications of the existence of the 

 fish are rare enough ; for every shark in the course of its 

 life casts off great numbers of teeth, which fall to the 

 bottom of the sea and become bedded in the deposit there 

 forming. 



The teeth are never anchylosed to the jaw, nor have they 

 any direct connection with it, but, as before mentioned, are 

 retained by being bedded in a very tough fibrous mem- 

 brane ; the nature of their fixation has been more exactly 

 described at another page (page 202). 



The sheet of fibrous gum slides bodily over the curved 

 surface of the jaw, continually bringing up from below 

 fresh rows of teeth, as was proved by Andre's specimen, 

 and it may be worth while to condense from Professor 

 Owen the description of the manner in which it was thus 

 proved that an actual sliding or rotation of the membrane 

 does really take place, and that the whole bony jaw itself 

 does not become slowly everted. The spine of a sting ray 

 had been driven through the lower jaw of a shark (Galeus), 

 passing between two (vertical) rows of teeth which had 

 not yet been brought into use ; when the specimen came 

 under observation the spine had remained in this situation, 

 transfixing the jaw, for a long time, as was evidenced by all 



