54 o SHARKS AND RAYS. 
verse ridge; the dorsal fins, which are situated far behind the pelvic pair, have 
no spines; and the caudal has no lower lobe. Fossil species are found from the 
Tertiary to the upper Jurassic. The allied Australian genus Trigonorhina differs 
in having the front nasal valves united, and forming a broad quadrangular flap. 
thornback skates (J g nat. size). 
Rhynchobatis The third genus of the family, which is represented by two 
species from the coasts of the tropical portions of the Indian Ocean, 
differs in that the first dorsal fin is placed above the pelvic pair, in the presence 
of a lower lobe to the caudal, and also in the form of the dental plate, which is 
