180 EVOLUTION OF ELASMOBRANCHS vi. 3- 



Permian, but in subsequent times they disappeared without leaving 

 descendants. 



4. Mesozoic sharks 



After flourishing in Palaeozoic seas the shark line seems to have 

 become nearly extinct during the Permian and Trias. During this 

 period there was probably little fish life in the sea and the stock 

 seems only to have survived by adopting a varied diet, including 

 invertebrate food. The protoselachian or heterodont sharks of this 

 period had two types of tooth, pointed ones in front and flattened 

 ones, for crushing molluscs, behind. Heterodontus, the Port Jackson 

 shark of the Pacific, is a surviving form having a dentition of this 

 type. 



There is total cleavage of the yolk of the egg. The meroblastic form 

 typical of modern elasmobranchs and teleosts was therefore a rela- 

 tively late development and other survivors of the mesozoic period 

 besides Heterodontus also show holoblastic cleavage (pp. 184-236). In 

 later Triassic times sharks again became more abundant, and this 

 agrees with the presence of numerous bony fish types, on which they 

 presumably fed. Some of the Triassic sharks still possessed a hetero- 

 dont dentition (*Hybodus), though otherwise much like the modern 

 forms. 



In Jurassic times or earlier, however, the sharks divided into the 

 main lines that exist today. In the suborder Pleurotremata or true 

 sharks the teeth all became sharp and the animals swift swimmers. In 

 the suborder Hypotremata, on the other hand, the teeth remained 

 flattened and sometimes became highly specialized for a mollusc- 

 eating diet (Fig. 1 14), producing the flattened bottom-living creatures, 

 the skates and rays. The stages of this transition can be followed, and 

 some of the intermediate types still exist. Thus in Rhifiobatis, the 

 banjo-ray (Fig. in), the pectoral fins are enlarged but still distinct 

 from the body. Almost identical creatures have been found in Jurassic 

 rocks. It is probable that several separate lines showed this flattening 

 of the body. 



5. Modern sharks 



The Pleurotremata may be divided into three divisions all dating 

 from the Jurassic. The Notidanoidea show many primitive features, 

 such as an amphistylic jaw, the presence of six or seven gill-slits, and 

 an unconstricted notochord. Hexanckus and Heptranchias, are long- 

 bodied, slow-moving sharks from warm waters. They are viviparous 



