64 THE VERTEBRATE SKELETON. 



archipterygium. In most cases they are sharply marked off 

 from the body and lie almost at right angles to it ; but in the 

 Rays they have the form of lateral expansions in the same 

 plane as the body, from which they are not sharply marked 

 off. The pelvic fins in the male bear long grooved cartilaginous 

 rods which are accessory copulatory organs or claspers. 



There are two principal groups of Selachii, the Squalidae 

 or Sharks aijd Dogfish, and the Batoidei or Skates and Rays. 

 The Squalidae have the shape of ordinary fish, the pectoral 

 fins are vertically placed and the body ends in a powerful 

 heterocercal tail. The Batoidei have flattened bodies owing 

 to the great size and horizontal position of the pectoral fins. 

 The tail is long and thin and is often armed with spines. 

 The teeth in Selachii differ much in character in the different 

 forms, and are always arranged in numerous rows. They are 

 generally pointed and triangular or conical in the Squalidae, 

 while in the Batoidei they are often broad and flattened. 



Suborder (4). ACANTHODII. 



The fishes included in this group are all extinct and 

 in some respects are intermediate between Elasmobranchii 

 and Gaiioidei. The body is elongated and closely covered 

 with small scales consisting of dentine enamelled at the sur- 

 face. The notochord is persistent and the calcification of the 

 endoskeletal cartilage is only superficial. The tail is hetero- 

 cercal. The jaws bear small conical teeth, or in some cases 

 are toothless. The skeleton of all the fins differs from that of 

 modern Elasmobranchs in having the cartilaginous radialia 

 much reduced, and the fins are nearly always each provided 

 with an anterior spine, which except in the case of the pec- 

 toral fins is merely inserted between the muscles. These spines 

 are really enormous dermal fin-rays ; the pectoral fin-spine is 

 articulated to the pectoral girdle. 



The suborder includes many well-known extinct forms like 

 Acanthodes and Diplacanthus ; it ranges from the Devonian to 

 the Permian. 



