PLAGIOSTOMI. 



135 



condition, unique in the vertebrate sub-kingdom, have large 

 and massive teeth (figs. 53-6). The smaller teeth of the Rhina 

 are adapted for crushing : but in the species of Rata, Guv., 

 they have the middle or one of the angles of the crown pro- 

 duced into a sharp point. In all genera of the ray tribe, 

 whatever the diversity of size and shape of the teeth, they are 

 placed in several rows, and succeed each other uninterruptedly 

 from behind. 



The modification of the plagiostornous type of teeth, for 

 the purpose of crushing alimentary substances, is most 

 complete in the Myliobatidw. A view of this armature of 

 the mouth, as seen from behind in the Myliobates aquila, is 

 given in fig. 53. Both jaws are covered with a pavement of 

 broad teeth, having a flat grinding surface. To the genus 

 Myliobates, as now restricted, certain fossils from the London 

 clay of Sheppy (Myliobates toliapicus, Ag., fig. 54) belong. 



Fig. 54. 



Myliobates toliapicus. 

 (Eocene, Sheppy). 



Fig. 55. 



Zijgobates Woodivardi. 

 (Miocene). 



In Zijgobates (fig. 55), the middle series of teeth is less 

 broad ; and a narrower series is interposed between the 

 middle and the small lateral teeth. Existing rays shewing 

 this modification are found in Brazilian seas ; fossil teeth of 

 this genus, e.g., Zygobates Woodwardi, Ag. (fig. 55), occur in 

 the tertiary crag (probably miocene) of Suffolk, and in the 

 miocene mollasse of Switzerland. 



When the teeth form broad transverse undivided plates, 



