RAJIFORMES 



167 



uniform, or some middle rows may be enlarged (Figs. 131, 132). In 

 Ceratoptera they are absent from the upper jaw. 



Dicerobatis (Cephaloptera), Blainv., Ceratoptera, M. and H., and 

 fihinoptera ; warm seas ; the latter to Eocene, Europe. 



Family PSAMMODONTIDAE. Known only from fossil teeth, which 

 are transversely elongated, flattened, and adapted for grinding. They are 



Rhinoptera Woodwanh, Ag. 

 (After Owen.) 



Lower Jaw. 

 Fio. 131. 



Teeth of Khinoptera polyodon, Gthr. 

 (After Giinther.) 



FIG. 133. 



Tooth of Pdyrhizodus pusilltis, 

 M'Ooy ; Carboniferous Limestone, 

 Armagh. (After M'Coy.) c, outer 

 grinding surface ; r, basal root. 



closely set in longitudinal rows, somewhat as in the Myliobatidae, with 

 which this family is generally associated. 



Gopodus, Davis, Psammodus, Ag. ; Lower Carboniferous, Europe and 

 N. America. Archaeobatis, Newb. ; Lower Carboniferous, N. America. 



IXCERTAE SEDIS. 



Family PETALODONTIDAE. These palaeozoic Selachians are chiefly 

 known from remains of the dentition, which is much specialised. In the 

 case of Janassa it has been ascertained that the body was depressed and 

 expanded, with large pectoral fins reaching forward and perhaps fusing with 

 the head (Miinster, Jaekel [240]). The jaws were wide and strong, 

 bearing powerful grinding teeth set one behind the other in regular rows. 

 Apparently the old teeth remained to support the newer growing up 

 from behind. As a rule, each tooth has a distinctly marked crown and 

 root. In Polyrhizodus the root is subdivided (Fig. 133). The affinities 



