LIVING MILLSTONES 159 
with in the whole animal kingdom) excel their artificial 
substitutes in that they never wear out; being renewed 
either by the development of new teeth on the inner or 
hinder aspect of the cylinder, or by vertical successors 
replacing the individual teeth from below or above. 
And now that the dental millstones of the rays have been 
mentioned, it will afford a convenient starting-point for a 
brief survey of some of the most remarkable types of 
structure presented by these curious organs. 
The teeth of rays always form a pavement-like structure, 
of which the component elements are arranged in straight 
longitudinal rows, although they sometimes likewise show 
a quincunxial mode of arrangement. The individual teeth 
are not replaced by vertical successors ; but, being in the 
form of a half-cylinder, as those in front become worn 
down, the whole series is pushed forwards, and new teeth 
are developed on the hinder margin of the cylinder. The 
supreme development of a dental structure adapted for 
crushing in this group occurs in the family of the eagle- 
rays (Mylhobatidae), in which the millstone of each jaw 
forms a perfect semi-cylinder or plate, made up of flat- 
crowned prismatic teeth united at their edges, often so as 
to constitute a mosaic-like pavement. No piece of modern 
machinery can be better adapted for crushing hard sub- 
stances than are these beautiful ivory cylinders and 
plates, the crushing power of which, when worked by the 
strong jaws, must be enormous, and sufficient to grind 
the strongest shell that can be introduced between them 
to powder. Although in all cases pavement-like, the 
millstone differs considerably in the different species in its 
structure. 
As an illustration of the group, we may take one of the 
millstones of the beaked eagle-rays (R/imoptera). Here the 
