60 CESTRACIONTS. 



grinding surface, with a great breadth of crown. It is of one of those 

 teeth of the typical Psammodi which I have submitted to microscopic 

 examination. A transverse section of the tooth of this genus pre- 

 sents the appearance, under a moderate magnifying power, as if it 

 were composed of close-set coarse tubes, the areas of which were thus 

 exposed. Such a section, viewed with a power of 400 diameters, 

 shows that these tubes are surrounded by concentric lamellae, like the 

 Haversian canals ; and that these lamellae, and the clear interspace, 

 which is generally equal to the thickness of the lamellae, are permeated 

 by minute irregularly disposed tubes, which anastomose in the clear 

 interspace, and open into extremely minute cells, scattered through 

 that part. A longitudinal section of the same tooth shows, in many 

 places, the whole course of the canals; they run nearly perpendicularly 

 to the convex superficies of the tooth, and, consequently, incline 

 outwards at the sides of the section. They lie nearly parallel with 

 each other, with interspaces equal to from six to eight times the 

 diameter of their area, and branch dichotomously once or twice in 

 their course. Each canal is surrounded by concentric layers of 

 a dark colour, encroaching upon one-third of the interspace, which 

 thus presents two semi-opake streaks and one intermediate clear 

 line ; the whole of these interspaces is perforated by the irregular 

 wavy, branched, anastomosing calcigerous tubes. The terminations 

 of the medullary canals near the periphery of the tooth are slightly di - 

 lated, and give off in every direction calcigerous tubes corresponding to 

 those in the interspaces of the canals. The calcigerous tubes, at this part 

 of the tooth, run obhquely along the side of the medullary canal, from 

 which they are continued, for a short distance before they pass off 

 through the concentric layers into the clear interspace ; to this 

 structure is due the appearance, which the medullary canal presents, 

 of being composed towards its termination of a fasciculus of spirally 

 twisted tubes. I have not observed this structure in any part of the 

 medullary canals of the tooth of the Ptychodus, which further differs 

 from that of Psammodus in the more frequent bifurcation of the medul- 

 lary canals, and in the smaller extent of their opake concentrically- 

 laminated walls. The structure of the tooth of Psammodus, like that of 



