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Order 2. — Pleuracantiiei (Ichthyotomi). 



Extinct Shark-like Elasmobranchs in which the skeleton of the 

 pectoral fin was constructed on the type of the so-called archi- 



pterygium, i.e. consisted of an 

 elongated, segmented central axis 

 bearing two rows of jointed rays. 

 The notochord was persistent ; in- 

 tercalary cartilages were present 

 in addition to neural and haemal 

 arches. The caudal fin is diphy- 

 cercal. Claspers were present. 

 There was no opercular fold, and 

 the teeth resemble those of other 

 Elasmobranchs. Placoid scales are 

 not known to have been present, 

 but the skull is protected by roof- 

 ing dermal ossifications. 



This order, like the last, in- 

 cludes only one satisfactorily 

 known genus — Plenracanthus (Fig. 

 828) — of Carboniferous and Per- 

 mian age. 



Order 3.— Acanthodei. 



Extinct Elasmobranchs (Fig. 829) 

 having the anterior margin of each 

 fin supported by a stout spine. 

 The tail is heterocercal. There 

 were no claspers. There is a 

 placoid exoskeleton of small den- 

 ticles. An operculum was not 

 present. The notochord was per- 

 sistent, with neural and haemal 

 arches. Calcified plates are pre- 

 sent in relation to the jaws and 

 to the roof of the skull. The 

 teeth are few and large, numer- 

 ous and minute, or altogether 

 .absent. The lateral line was in 

 the form of an open groove. 



Order 4. — Selachii. 



Living and extinct Elasmo- 

 branchs in which the skeleton of 

 the paired fins is never of the nature of an archipterygium. The 

 notochord is more or less completely replaced by vertebrae, and there 



