LIVING MILLSTONES 161 



depressions in the lower millstone. It is difficult to 

 conceive a machine better adapted for crushing than is 

 presented by the jaws of the beaked rays. 



Of a much less powerful type are the millstones of the 

 ordinary rays or skates (RaiidaeJ- of our own coasts; and 

 among these the common thornback {Rata clavatd) presents 

 a very remarkable condition, since the individual teeth take 

 the form of obtuse knobs in the female, whereas in the 

 male the centre of each of these knobs acquires a sharp 

 recurved point. Since everything in nature has a meaning, 

 it would seem a fair inference that there must be some 

 important difference between the food of the male and 

 female thornback, but I have not come across any obser- 

 vations bearing upon the subject. 



Among the fossils to be obtained occasionally from the 

 workmen in large chalk-pits are teeth in the form of convex 

 quadrangular bosses, the marginal portion of which consists 

 of a broad granular area, while the centre is occupied by a 

 variable number of bold ridges, or folds, between which are 

 often irregular knobs. It is from these ridges that the fish 

 takes the name of Ptychodus. For a long time it was un- 

 certain how these teeth were arranged, but careful comparison 

 of a number of more or less incomplete series in situ has at 

 length solved the problem. In the lower jaw the complete 

 millstone was formed by a median row of large teeth, 

 on each side of which were six or seven other rows 

 composed of teeth gradually decreasing in size from the 

 centre to the margin. In the upper jaw, on the other 

 hand, there was a central row of small teeth, flanked 

 on each side by a row of large ones, externally to which 

 came a series of rows gradually diminishing in size. From 

 this mode of arrangement it is inferred that Ptychodus was 

 a ray; and the whole dental structure is as remarkable 



ii 



