299 



The crown forms a prominent cone with evenly sloping sides, and with a 

 transversely oblong square base narrowing a little posteriorly. The posterior 

 sinus of the crown comports in its height and breadth with the proportions 

 of the former. The principal ridges of the surface of the crown cross the 

 summit and posterior slope transversely. Descending, they branch in a 

 divergent mariner and anastomose, so as to form a comparatively coarse retic- 

 ulation, extending to the borders of the crown. The reticulation covers the 

 anterior slope of the crown and the sinus posteriorly. The direction and 

 arrangement of the ridges resemble those in the European Ptychodus decur- 

 rens, but in this the principal ridges are much coarser and more widely 

 separated. The root is mutilated in the specimen. The transverse diam- 

 eter of the crown is 14 lines ; its fore and aft diameter and its height about 

 1 inch. 



Figs. 15, 16, Plate XVIII, represent two views of a small tooth, which 

 may probably belong to the same species. It is unsymmetrical, and is worn 

 away at the summit of the crown. The latter is proportionately less promi- 

 nent than in the large tooth, but has its ridges arranged in the same general 

 manner. The root is very thick in comparison with the size of the tooth. 

 The transverse diameter of the crown is 1\ lines; the fore and aft diameter 

 \ inch. 



Of five remaining specimens, one is a smaller and unworn tooth nearly 

 like that last described. Its crown is '5 lines wide and 4 lines from before 

 backward. 



The specimen represented in Fig. 17 is more symmetrical, and nearly 

 resembles in shape the smaller symmetrical teeth of Ptychodus Mortoni, as 

 represented in Figs. 13, 14. The apex of the crown is not so pointed, but is 

 prolonged fore and aft in an acute ridge, and the rugae of the surface are not 

 convergent, but cross the summit in the usual transverse manner of the other 

 teeth. The breadth of the crown in this specimen is 4£ lines ; the antero- 

 posterior diameter 3f lines. 



The remaining teeth, of which the largest is represented in Fig. 18, have 

 a transversely ovoidal crown slightly elevated to one side of the center. The 

 surface is crossed by rugse in the same manner as in the large teeth. 



