130 PALAEONTOLOGY. 



ridges. With teeth of this form are sometimes found others 

 of smaller size, with more convex rounded crowns, doubtless 

 forming the extremes of the multiserial pavement which, as 

 in modern sharks and rays, covered the 

 broad jaws of the Ptychodonts. In my 

 " Odontography,"* I have pointed out the 

 resemblance of the teeth of Ptychodus to 

 those of Rhina, and Sir P. Egerton informs 

 me that the fin-rays of Ptychodus shew the 

 same affinity. But the principle of the 

 " more generalised structure " was mani- 

 fested in this, as in most contemporary 

 forms. The large dorsal spines found asso- 

 Fig. 44. ciatecl with the above-described teeth are 



Ptychodus latissimus. longitudinally grooved, and resemble those 

 of the Cestraciont family of sharks. All 

 the specimens and species referable to this genus have been 

 found in the cretaceous strata. The Devonian Ctenodus, Petalo- 

 dus, Ghomatodus, and Petrodus, of the carboniferous limestone, 

 and Thectodiis of the Keuper, are provisionally referred to 

 the Cestraciont family. 



Family II. — Hvbodontes. 



Teeth referable to the genus Hybodus occur in all the 

 secondary rocks from the trias to the chalk inclusive. 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 " secondary." 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 



* Vol. i., p. 44 



