CESTRACIONTS. 



53 



opacity occasioned by the earthy matter deposited in them from their 

 peripheral extremities to within a short distance of tlie base of the 

 tooth. The fine calcigerous tubes, from the same cause, appear as 

 dark Unes in the interspaces of the medullary canals, and as they pass 

 into the enamel-like superficial layer. 



In Plate 13 is exhibited a portion of two of the terminal medul- 

 lary loops, with the calcigerous tubes continued from them as seen 

 under a magnifying power of 600 diameters. The interspaces of the 

 medullary canals are traversed throughout the substance of the tooth 

 by calcigerous tubes, which have a general direction vertical to the 

 medullary canal from which they proceed. 



The calcigerous tubes present, at their origin, a diameter, gene- 

 rally, of s^'^th of an inch. They are more ramified, and have a more 

 undulatory course than the medullary canals, especially those which 

 occupy the interspaces of these canals, where they form a moss-like 

 network, in the meshes of which are minute cells, with which the finest 

 branches of the calcigerous tubes are in communication. The cal- 

 cigerous tubes of the grinding surface of the tooth have a general 

 direction, vertical to that surface, and the enamel-like coating is 

 formed principally by the finest terminal branches of these tubes, 

 imbedded in a transparent and apparently structureless matrix. 

 Under a power of rVth inch focus, longitudinal series of irregular and 

 partially confluent rhomboidal cells are discernible in many parts of 

 this external layer. 



The jaws of the Cestracion, like those of the other sharks, 

 exhibit the teeth in various stages of formation. At the innermost 

 extremity of some of the rows, may be seen a small, flat, punctate, 

 milk-white, calcareous plate, of a friable textare, resulting from the 

 recent deposition of the earthy particles in the microscopic cells and 

 tubes of the superficies of the formative matrix ; the only part which 

 as yet is developed from the common mucous and ligamentous basis 

 of the teeth. The rest of the matrix is progressively added until 

 the tooth acquires its full size ; the deposition of the calcareous salts 

 proceeding from the crown to the base simultaneously along the whole 

 breadth of the tooth. 



The process of dentification is here most clearly seen to be one 



