58 CESTRACIONTS. 



figs. 1 and 2) presents a quadrangular form, and consists of a root 

 and a crown ; a wide but shallow groove forms the line of demarca- 

 tion between these parts. The root, which presents a coarse, porous, 

 osseous texture, is flat below, and gradually enlarges towards the 

 crown. The crown suddenly expands above and beyond the root, 

 especially beyond its anterior part. The anterior and two lateral 

 surfaces of the crown are convex, the posterior surface (fig. 1) is 

 concave, apparently for the reception of the convexity of the succeed- 

 ing tooth. The broad upper surface of the crown is more or legs 

 convex, and sculptured with minute tubercles and wrinkles at the 

 circumference, and with large angular transverse ridges in the centre. 

 These ridges are separated by wide grooves, and generally have their 

 extremities bent towards the anterior part of the tooth ; the surface 

 of the crown is smooth and polished like enamel. In fig. 2, the 

 sculptured grinding surface of a tooth of the Ptychodus latissimus is 

 represented of the natural size. 



The texture of the tooth of the Ptychodus, examined in a longi- 

 tudinal vertical section at half an inch focus, as in PI. 18, presents a 

 congeries of medullary and calcigerous tubes, having the same general 

 arrangement as in the Cestracion. The medullary tubes are, however, 

 relatively smaller ; they proceed from the coarse canals of the osseous 

 base in a nearly straight and parallel direction towards the surface 

 of the crown, diverging from each other, and branching dichoto- 

 mously, so as to maintain a direction vertical to the surface towards 

 which they proceed. Their interspaces are pretty regular, and about 

 five or six times the diameter of the canals themselves. They are 

 surrounded by concentric lamellae (fig. 2, PI. 19), and send off through 

 the whole of their course numerous minute calcigerous tubes ; these 

 are transverse to the medullary canals near the base of the crown, and 

 come off at an acute angle as they approach nearer the summit, close 

 to the surface of which the medullary canals resolve themselves into 

 fasciculi of calcigerous tubes (fig. 1, PI. 19); in a few places, two 

 contiguous medullary canals anastomose, and form a loop, with the 

 convexity directed towards the surface of the tooth ; one of these 

 loops is shown in fig. 1, PI. 19. The calcigerous tubes are more 



