42 SHARKS. 



canals approach the apex of the tooth. The calcigerous tubes 

 are characterized by their frequent branching and inosculation ; 

 the branches go off generally at right angles to the trunk, which 

 they nearly equal in size ; these quickly anastomose and again 

 send off smaller branches which similarly anastomose with others 

 of corresponding size, until the terminal tubes are, for the most 

 part, lost in a series of minute calcigerous cells which form the 

 boundaries of the system of calcigerous tubes developed from 

 each medullary canal. Some of the terminal tubes of contiguous 

 systems anastomose across this boundary. Each of the systems of 

 calcigerous tubes represents a separate denticle, of a prismatic figure, 

 exhibiting in transverse section generally a more or less regular hexa- 

 hedron. Towards the point where two contiguous medullary canals 

 inosculate the terminal calcigerous tubes of each system begin to 

 exhibit more frequent anastomoses, and the boundary hne is thus 

 gradually obliterated ; the letters a, b, and c, pi. 9, fig. 2, exhibit two 

 systems of calcigerous tubes blending together near the point where 

 the two contiguous medullary canals were about to inosculate. 



The reticulate arrangement of the calcigerous tubes is more, and 

 the radiated one less, conspicuous in the rostral teeth of Pristis than 

 in the teeth of any other species which I have yet examined. The dia- 

 meter of the calcigerous tubes at their origin is ~th of an inch, their 

 terminal branches may be traced to the minuteness of ^oioo*^ of an 

 inch. 



In the embryo of a Pristis six inches long, to which the umbilical 

 chord was still attached, I found a series of depressions in the skin 

 along the margins of the rostral prolongation corresponding in num- 

 ber and relative position with the future teeth ; and at the bottom 

 of each of these dermal follicles, there was a papilla which formed 

 the apex of a pulp, whose base had already begun to penetrate the 

 cartilaginous plate of the rostrum. The pulp had the usual dense 

 and unyielding external ' membrana propria,' and its apex was covered 

 by a continuation of the tegumentary follicle of extreme thinness, 

 but there was no true capsule or enamel organ. The calcareous par- 

 ticles had not begun to be deposited in the tissue of the pulp. 



The teeth of the young specimen, of which the head and saw are 

 figured in PI. 8, fig. 1, were fully calcified, and except that the num- 



