160 MOSTLY MAMMALS 
millstone is in the form of a semi-cylinder, consisting of five 
or more rows of teeth; a very usual number being seven. 
Generally the teeth of the middle row are the widest; those 
of the rows on either side being considerably narrower, 
while the two or three marginal rows on each side may be 
compared to the tesserz in a mosaic pavement. A further 
development of the same type is exemplified by the typical 
eagle-rays (Myliodatis), in which the middle row of teeth 
in the millstone becomes still wider, while the three lateral 
rows on each side are reduced to the condition of hexagonal 
tesserae. Moreover, whereas in the species of Rhinobatis 
both millstones are in the form of half-cylinders, in 
Myhobatis the upper one alone retains this form, the lower 
being a flattened plate. The culmination of this type of 
sculpture is displayed in the rays belonging to the allied 
genus Aéfobatis, in which both upper and lower millstones 
are flat and composed only of the middle row of teeth, 
which are of great width; the lateral rows having com- 
pletely disappeared. The existing representative of this 
genus is not very large (for a ray), seldom, if ever, measuring 
more than about five feet across; but some of its extinct 
predecessors must have been monstrous fish, as the teeth 
measure some five or six inches in diameter. 
Quite a different type of dental armature is presented 
by the millstones of the beaked rays (Rhznobatidae). 
Here the teeth take the form of closely packed diamond- 
shaped knobs, arranged in an alternating manner, so that 
although they present longitudinal rows, yet they also 
show oblique series, so as to give rise to a quincunxial 
pattern. Then, again, the entire millstone in each jaw is 
thrown into a series of undulations, so that the upper 
one exhibits a large median boss, flanked by a pair of 
smaller undulations, which are received into corresponding 
