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 {Myliobatidae), 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 (Rkinoptera). Here the 



