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 tesserae in a mosaic pavement. A further 

 development of the same type is exemplified by the typical 

 eagle-rays (Myliobatis), 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 

 Myliobatis 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 Aetobatis, 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 (Rhinobatidae). 

 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 



