58 Heineich Engel. 



ridge which projects into the back part of the socket. The rostral 

 tooth is solid, as shewn in the longitudinal section of the one figured 

 in tab. 8, fig. 3; its base is slightly concave and porous, like the 

 section of a cane, but the pores are finer and more numerous. The 

 walls of the socket are formed by ossification of the rostral cartilage 

 to the adequate extent; but as undue weight, under any circum- 

 stances, and especially at the fore end of the fish would be a 

 cumbrous impediment to its motions, the intervening spaces between 

 the sockets are hollow and filled with a gelatinous medulla. A 

 large vascular canal traversed by branches of the facial artery, and 

 of the second division of the fifth pair of nerves, and inclosed in a 

 cellular and gelatinous tissue, runs parallel with the axis of the saw 

 along the back part of the alveoli, and supplies the materials for 

 the increase of the teeth, which are not shed and renewed like the 

 maxillary teeth, but grow with the growth of the body by constant 

 addition of fresh pulp-material progressively ossified at their base. 

 The structure of the rostral teeth of Pristis has the nearest 

 resemblance to that of the maxillary dental plates of Myliohates ; 

 they are traversed throughout b}?^ medullary canals which run 

 parallel to each other and to the axis of the tooth; but thej' ex- 

 hibit more frequent anastomoses and dichotomous sub-divisions than 

 in Myliohates. The diameter of the medullary canal is Vjooth inch, 

 and their interspaces Vöotli of an inch near the base of the tooth; 

 they are surrounded by concentric laminae which increase in number 

 as the 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 angle to the trunk, which they 

 nearl}'^ equal in size; these quicklj^ anastomose and again send off 

 smaller branches which similarly anastomose with others of corre- 

 sponding 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 hexahedron. 

 Towards the point where two contiguous medullary canals inosculate 

 the terminal calcigerous tubes of each system to begin exhibit more 

 frequent anastomoses, and the boundary line is thus gradually 

 obliterated : the letters a, b and c, tab. 9, fig. 2, exhibit two systems 



