SUB-CLASS I 



SELACHIl 



27 



Sub-Order C. ASTEROSPONDYLI. Hasse. 



Vertebral centra when completehj formed amphicoelous, strengthened by secondary 

 plates of calcified tissue which radiate outwards from the excessively constricted 

 primitive cylinder. Five gill clefts on each side. Two dorsal fins and one anal fin 

 present. Lower Carboniferous to Eecent. 



Family 1. Oestraciontidae. Agassiz. 



Dorsal fins each armed with a spine. Teeth numerous, mostly obtuse, never fused 

 into continuous plates ; several series simultaneously in function. Radiating plates of 

 the vertebral centra few and short ; vertebral column not ccdcified in the earlier forms. 

 Lower Carboniferous to Eecent. 



This, family is characteristic of the later Palaeozoic and the Mesozoic 



formations ; it is represented in existing seas solely by one genus, 



Cestracion. 



Orodus, Ag. (Fig. 49), known only by obtuse elongated teeth. The dental 



crown raised in the middle, and its surface marked with more or less yjrominent 



wrinkles, which rise from each long margin 



or from a median longitudinal crest. Base 



of tooth very stout. 0. ramosus, Ag. (Fig. 



49), and other species from the Carboniferous 



Limestone .of Britain, Belgium, and Eussia. 



Also numerous species in corresponding 

 American formations. 



Campodus, Kon. {Agas- 

 sizodus, St. J. and W. ; Arpagodus, Traut.). Teeth almost 

 identical with Orodus, but distinguished by buttressed 

 condition of the outer border of the crown. Symphysial 

 series greatly enlarged, and fused. Carboniferous. Edestus, 

 Leidy ; Campyloprion, Eastm. ; and Helicoprion, Karp., 

 known only by their arched or coiled symphysial dentition. 

 Carboniferous and Permian. 



Trisfychius, Ag. (Ptychacanthus, Ag,), (Fig. 50). Dorsal 

 fin spines with a few sharo longitudinal ridges distally, 

 three only- — or;ie upon the front margin and one on each 

 side — extending far downwards. Teeth probably as in 

 Sphenacanthus. T. arcuatus, Ag. Carboniferous. 



Sphenacanthus, Ag. Teeth rather high-crowned, with a 

 large principal cusp and smaller lateral cusps diminishing 

 in size outwards, more or less marked by superficial 

 wrinkles ; base depressed, forming a slight expansion back- 

 FiG. 50. wards. Notochord persistent. Dorsal fin spines marked 



A'^^'Dorsaifln-s'Tne'/l)' '^^^^ robust lougitudiual ridges, in part nodose ; posterior 



shagreen-scale (B), and denticles Small and uumerous, in two series, one upon each 



size. Lower^Carbord- lateral margin of the flattened or slightly concave posterior 



^^o^T).' ^""^^^""^ ^^^^'''■' face. Few minute comb-shaped shagreen granules. ^S'. 

 costellatus, Traquair sp., known by nearly complete fish 



from Calciferous Sandstones (Lower Carboniferous) of Eskdale, Dumfries. 



^ 



Orodus ramosus, Ag. Carboniferous Limestone ; 

 Armagh (after Davis), i/o. 





