PLEUROPTER YGI I. 



145 



The Elasmobranchii are the most ancient of all knowoi fishes. 

 They make their appearance in the Upper Silurian. They are 

 almost entirely active, carnivorous, predatory fishes and with 

 very few exceptions exclusively marine. 



The following is the classification adopted in this work : — 

 Order 1. Pleuropterygii (extinct). 



2. acanthodi 

 ,, 3. ichthyotomi ,, 



4. Selachii (Plagiostomi). 

 Suborder 1. Notidani. 



2. Squall. 



3. Raji (Batoidei). 



5. HOLOCEPHALI. 



Fia. 83. — Cladosdache. A pectoral, B pelvic fins x i : B basal somactids within the body- 

 wall, D dermal fin membrane, R peripheral somactids. Left border preaxial (after Dean, 

 from Woodward). J 



» Order 1. Pleuropterygii.* 



' With unconstricted notochord and heterocercal caudal fin. Paired fins 

 with unsegmented parallel radials, reaching to the edge of the fin. Eyes 

 with a circle of thin dermal plates. Male without claspers on the pelvic 

 fins. 



The skull is unknown, but the jaws are suspended by a slender hyo- 

 mandibular. The teeth have a principal cusp and several accessory 

 lateral cusps. They resemble teeth which have long been known from 

 the Carboniferous under the generic name Cladodus. There were certainly 



* B. Dean, Contributions to the Morphology of Cladoselache, Journ. 

 Morph., 9, 1894. Jaekel, Ueber Cladodus, Sitzungsb. d. Oesellsch. naturf. 

 Freunde, Berlin, 1892. R. Traquair, Geol. Magazine, 1888, p. 83. 



