SHARKS. 37 



ing with those of the subsequent calcigerous tubes. The formation of 

 the tooth commences by the deposition of earthy particles in the tough 

 external membrane of the pulp. I have been unable to recognize the 

 distinct arrangement of the hardening salts in this layer. It is trans- 

 parent, extremely dense, and forms the enamel-like polished coating 

 of the tooth J in sections of fully formed teeth, the finest terminal 

 branches of the parallel peripheral calcigerous tubes are lost in the 

 above clear enamel-like substance. When the enamel -like outer 

 layer of the apex of the tooth is completed, it is so easily detached 

 from the subjacent pulp that it might be readily supposed that there 

 was no organic connection between them. If, however, the so ex- 

 posed pulp be now examined with the microscope, and compared with 

 an uncalcified pulp, it is seen to be no longer covered with the 

 smooth dense membrane observable in the latter ; but the apical edges, 

 from which the enamel-like cap has been detached, appear villous 

 or flocular. It is obvious that the first shell of the tooth has been 

 neither transuded from the superficies of the external membrane of 

 the pulp nor has been deposited between that membrane and the 

 granular part of the pulp, but is due to a conversion of the external 

 membrane into a dense enamel-like bone. The formation of the body 

 of the tooth by deposition of earthy particles in pre-existing and 

 pre-arranged cavities is still more satisfactorily demonstrable. In 

 proportion as the formation of the tooth has advanced, the diffi- 

 culty of separating the calcified from the uncalcified portion of the 

 pulp is increased, and at the same time it becomes easier to detect 

 the continuation of the processes of the pulp into those medullary 

 canals which form so many separate centres of radiation of the plexi- 

 form calcigerous tubes. 



The application of the principle of dental development by con- 

 version and not by transudation, as illustrated in the dentition of 

 the shark, to the formation of the mammiferous tooth, is by no means 

 forced, but natural and obvious. In the ivory part of a simple mam- 

 miferous tooth there is a single medullary canal, the cavitns pulpi, and 

 a single system of radiated calcigerous tubes ; but the plan and prin- 

 ciple of formation is the same as in the shark. 



In proportion to the quantity of earthy matter deposited in the 



