36 SHARKS. 



the succeeding row, the papillee are larger, the cone broader and 

 flatter, and its apex is covered with a small cap of dense and glis- 

 tening dental substance which is readily removed ; though not with- 

 out displacement of part of the pulp, the granules of which, adherent 

 to the cavity of the displaced dental cap, are always readily recog- 

 nizable under the microscope. The third series of papillse, count- 

 ing from below in the lower jaw, have acquired the size and 

 shape of the future tooth, with the crenate edges well marked ; 

 half the tooth is completed, and its removal from the fleshy base 

 of the pulp cannot be effected without evident laceration of the 

 pulp ; when this is done under the microscope, the torn processes of 

 the pulp continued into the medullary canals of the new formed 

 tooth are plainly visible. The fourth tooth is completely formed, 

 as also the fifth and sixth, in the ascending series ; these progressively 

 diminish in size. The last or highest, which is first exposed on re- 

 flecting the thecal fold, and the first which was completed in the 

 order of development, consists of a simple cone, similar in form and 

 size to the apical third of the ordinary sized teeth below it ; yet 

 its growth is quite completed, and its base firmly attached to the 

 maxillary membrane. 



In a foetus of a Carcharias, three inches long, which had not lost 

 its external branchiae, the membranous groove between the jaw and 

 thecal fold was much shallower, and only two rows of papillse were 

 present on the maxillary membrane. The minute anterior teeth in 

 the more advanced foetus are doubtless developed from these primi- 

 tive papillse, which must be succeeded by others of progressively 

 larger size till the normal form and dimensions of the adult teeth 

 are attained. (1) 



The unossified pulps, examined with a high power, consist of semi- 

 opaque polyhedral granules or cells suspended in a clear matrix, and 

 the whole is inclosed in a tough transparent membrane which forms 

 the outer surface of the pulp. Beneath this membrane at the crenate 

 margins, the granules or cells are arranged in lines precisely correspond- 



(1) The foetal shark is peculiarly favourable for such comparisons, as it presents numerous 

 pulps and teeth in every stage of formation, easily detached and without violence from their 

 exposed situation, and of a flattened form well adapted for microscopical observation. 



