CESTRACIONTS. 59 



wavy than the medullary canals ; they quickly ramify, and sub-divide 

 to extreme minuteness in the interspaces, and finally terminate by 

 anastomosing with each other, either immediately or by the inter- 

 position of calcigerous cells. 



The intimate structure of the tooth of the Ptychodus differs from 

 that of the tooth of the Cestracion, in the medullary tubes being 

 narrower, the interspaces wider, and the terminal anastomosing loops 

 fewer. The calcigerous tubes, also, are relatively larger, more wavy, 

 and more branched. It differs from the structure of the tooth of 

 the Acrodus in the straighter course, and fewer divisions of the me- 

 dullary canals, and in the absence of the straight parallel superficial 

 series of calcigerous tubes. 



22.Psammodus. — Under this name, M. Agassiz had formerly asso- 

 ciated all those teeth of fishes " which combine a structure like that 

 which characterizes the teeth of the Cestracion,'' — that is to say, a crown 

 formed of small vertical tubes, — "with a surface of the crown more or 

 less smooth, and presenting only that sanded or punctate character 

 which results from the structure of the crown." 



Here, however, I must observe that neither in the teeth of 

 Psammodus nor in those of Cestracion have the punctate impressions 

 of the enamelled surface any relation with the medullary canals or 

 the large visible vertical tubes to which the learned Ichthyologist just 

 quoted refers. These tubes always terminate at a short distance 

 from the surface of the tooth, either by anastomosis or by subdivision 

 into other tubes of such extreme minuteness that the combined 

 diameters of five hundred of them would barely equal the breadth 

 of a single superficial punctation. These impressions on the teeth of 

 the Psammodi, like the transverse ridges of those of the Ptychodi, are 

 consequences of the conformation of the original matrix, and can be 

 regarded only as adaptations conformable with the habits and food 

 of the extinct species ; and as they are not due to a certain tubular 

 structure, so neither can they be viewed as evidence of such structure 

 when it has not otherwise been proved to exist. 



The term Psammodus is now restricted by M. Agassiz to the 

 extinct fishes, the teeth of which combine a broad, flat, punctate, 



