xm 



PHYLUM CHORDATA 



163 



fold of skin. The teeth are of the nature of placoid denticles, 

 lateral line was represented by an open groove. 



This order comprises only^ one 

 known representative Cladoselache 

 (Fig. 838) from the lower Car- 

 boniferous rocks of America. 



ORDER 2. PLEURACANTHEI 



(ICHTHYOTOMl). 



Extinct Shark-like Elasmobranchs 

 in which the skeleton of the pectoral 

 fin was constructed on the type of 

 the so-called archipterygium, i.e., 

 consisted of an elongated, segmented 

 central axis bearing two rows of 

 jointed rays. The notochord was 

 persistent ; intercalary cartilages 

 were present in addition to neural 

 and haemal arches. The caudal fin 

 is diphycercal. Claspers were pre- 

 sent. 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 roofing 

 dermal ossifications. 



This order, like the last, includes 

 only one satisfactorily known genus 

 Pleuracanthus (Fig. 839) of Car- 

 boniferous and Permian age. 



The 



ORDER 3. ACANTHODEI. 



Extinct Elasmobranchs (Fig. 840) 

 having the anterior margin of each 

 fin supported by a stout spine. The 

 tail is heterocercal. There were no 

 claspers. There is a placoid exo- 

 skeleton of small denticles. An 

 operculum was not present. The 

 notochord was persistent, with neural 

 and haemal arches. Calcified plates 

 are present in relation to the jaws 

 and to the roof of the skull. The teeth are few and 

 and minute, or altogether absent. The lateral line 

 of an open groove. 



large, numerous 

 was in the form 



L 2 



