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 (Raitdae) of our own coasts; and 
among these the common thornback (Raza clavata) 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 im seu 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 
