XIII 



PHYLUM CHORDATA 



155 



forms. The notochord was persistent : there were 



cartilages. The caudal fin was 



strongly heterocercal. Claspers 



were absent. The gill-openings 



were apparently protected by a 



fold of skin. The teeth were of 



the nature of placoid denticles. 



The lateral line was represented 



by an open groove. 



This order comprises only one 

 known representative Cladose- 

 lache from the lower Carbon- 

 iferous rocks of America. 



ORDER 2. PLEURACAXTHEA. 



no intercalary 



Extinct Shark-like Elasmo- 

 branchs in which the skeleton 

 of the pectoral fin was con- 

 structed on the type of the 

 archipterygium, i.e. consisted of 

 an elongated segmented central 

 axis bearing two rows of jointed 

 rays. The notochord was per- 

 sistent, but intercalary cartilages 

 were present. The caudal fin 

 was diphy cereal. Claspers were 

 present. There was no opercular 

 fold, and the teeth resembled 

 those of other Elasmobranchs. 

 There were no placoid scales, but 

 the skull was protected by roofing 

 dermal bones. 



This order, like the last, in- 

 cludes only one satisfactoril y 

 known genus Pleuraca nthus 

 of Carboniferous and Permian 

 age. 



ORDER 3. ACAXTHODEA. 







Extinct Elasmobranchs having 

 the anterior margin of each fin 



supported by a stout spine. The tail was heterocercal. There 

 were probably membrane-bones on the roof of the skull. The 



