24 COPE 



with or without smaller lateral cusps. The Cladodontidse 

 embrace the largest and most formidable species, and -be- 

 long to the Carbonic period. The Ichthyotomi are all con- 

 fined to the Carbonic system. 



The Selachii (sharks and rays) present two lines of re- 

 lation, or suborders, which dififer as follows : 



Vertebrae, when developed, having the con- 

 centric laminae predominating over the 

 radiating laminae ; anal fin absent ; Tedospondyli. 



Vertebrae when developed, with the radiat- 

 ing laminae predominating over the con- 

 centric ; anal fin present ; Aster ospondyli. 



In the Tedospondyli the majority of living types have 

 the body depressed, so that the branchial fissures are on 

 the inferior surface. This type is seen in the skates, saw- 

 fishes, and rays. In the Aslerospondyli, on the other hand, 

 the branchial fissures are lateral. There are several 

 families of Tectospondyli, which appeared at different 

 periods of geological time. They are as follows : 



I. Snout unarmed. 



A, The crowns of the teeth closely overlapping 

 each other like shingles. 



Edges of crowns forming a grinding face ; (1) Petalodontidas. 



AA, The crowns of the teeth not overlapping. 



/?. Summits of crowns forming a grinding sur- 

 face. 



(2) Psammodontidse. 



(3) Rajidse. 



(4) Rhinobatidse. 



(5) Trygonidx. 



(6) Myliobatidse (eagle rays). 



j9/3. Summits of crowns elongate cusps. 



