1 08 PALAEONTOLOGY 



and under different conditions, in the osseous fishes. They 

 indicate, in the present cartilaginous species, a diet of a lower 

 organized character than in the true sharks ; and a correspond- 

 ing difference of habit and disposition is associated therewith. 

 The testaceous and crustaceous invertebrate animals constitute 

 most probably the principal food of the Cestracion, as they 

 appear, by their abundant remains in secondary rocks, to have 

 done in regard to the extinct Cestracionts, with whose fossil 

 teeth they are associated. 



From their mode of attachment, these teeth would become 

 detached from the jaws of the dead fish, and dispersed in the 

 way above described ; and it is by such detached fossil teeth 

 that we first get dental evidence of the Cestraciont family in 

 former periods of the earth's history. 



The teeth of the Hybodonts are conical, but broader and 

 less sharp than those of true sharks. The enamel is strongly 

 marked by longitudinal grooves and folds. One cone is larger 

 than the rest, and called the "principal ;" the others are " second- 

 ary." In one genus (Cladodus, Ag.), the secondary cones go 

 on enlarging as they recede from the principal cone ; and teeth 

 of this genus, referred by Eichwald to the Hybodus longi- 

 conus, have been discovered in the old reel sandstone in the 

 vicinity of Petersburg. 



In the Orodus, the cones are more compressed, trenchant, 

 and distinct from the body of the 

 tooth than in Hybodus ; but they 

 present a principal and secondary 

 cones. Fig. 28 is a tooth of the 

 _ , . ' ' , . Orodus ductus (Ag.), from the car- 



Orodus anehts (tooth). . 



(Carboniferous.) boniferous beds near Bristol The 



0. porosus and 0. compressus are from deposits of similar age 

 near Armagh. 



If fig. 29 be compared with fig. 27 a, it would seem as if 

 the several teeth of each oblique row in Cestracion had been 



