SUB-CLASS I 



SELACHII 



19 



Order 1. PLBUROPTBRYGII. Dean.^ 



Endoskeleton well calcified. Notochord apparently not segmented. - Pterygo- 

 quadrate arcade movably articulated with the cranium. Orbit surrounded with a 

 ring of tliin plates of dentine. Paired fins supported hy unjointed, parallel radial 

 cartilages, ivhich may he partly fused at the hase, but extend directly outwards to the 

 edge of the fin membrane. Pelvic fins of male without daspers. 



According to Bashford Dean and Smith Wood ward, the Pleuropterygii 

 include the most primitive known representatives of the Selachii. Their 

 vertebral axis seems to have remained unsegmented, and in the typical genus 

 Cladoselache, the tail is strongly heterocercal. Their teeth are of the form 

 commonly known as Cladodus (Fig. 35) in the Carboniferous formations, and 

 several series must have been functional at one and the same time. 



Cladoselache, Dean (Fig. 34). The only well-defined genus hitherto dis- 

 covered. The form is elongated and round-bodied, with a short blunt snout and 



Fig. 34. 



Restoratiou of Cladosela,che newberryi, Dean. Cleveland Shale (Upper Devonian) ; Ohio. An anterior 

 dorsal fin has been discovered since this illustration was inade (after Dean). 



forwardly placed eye. Circumorbital ring consisting of several series of thin, 



irregularly quadrangular small plates. Mouth terminal, and hyomandibular 



cartilage slender. Teeth largest, longest, and most acutely pointed at the 



Symphysis of the jaw, smallest and shortest at the angle 



of the mouth. Each tooth with one principal cusp and a 



variable number of lateral cusps, fixed on the anterior 



border of a horizontally expanded base ; the base of each 



tooth overlapped by its successor behind. Paired fins 



lappet-like, the pelvic pair with somewhat extended base 



line. Two low dorsal fins, without spine. Anal fin not 



yet observed. A short dermal expansion forming a 



horizontal keel on each side of the caudal pedicle just in boniferöus 



advance of the strongly heterocercal caudal fin. Skin more ^™jf ' ^'^^^''^ ^^^^«^ 



or less armoured with minute lozenge-shaped denticles, 



which are apparently not enamelled. C. kepleri, C. fyleri, and other species 



sometimes two metres in length, from the Cleveland Shale of Ohio. 



The teeth termed Cladodus (Fig. 35) occur isolated in the Lower Carboni- 

 ferous of England, Scotland, Ireland, Belgium, Eussia, India, and North 

 America. They are also recorded from the Upper Carboniferous both of 

 Europe and North America, but it is difficult to distinguish them from those of 

 the genus Sphenacanthus (p. 27). It is possible, however, that these teeth do 



^ Dea7i, Bashford, Contribiitions to tlie Morphology of Cladosdadie (Jouru. Morpliol. vol. IX.), 

 1894.— A new Cladodout from the Ohio Waverly (Trans. New York Acad. Sei. vol. XIII.), 1894. 

 —The Finfold Origin of the Paired Limbs (Anat. Anz. vol. XI. No. 22), ]896. 



Fig. 35. 



Cladodus striatns, Ag. 

 Tooth, natural size. Car- 

 Limestone ; 



