1 1 PALEONTOLOGY 



fossil. No tooth referable to the genus has been found in any 

 tertiary stratum. 



The genus Ptijclwdus is founded on teeth usually of large 

 size, and of a more or less square form (fig. 31). The crown 

 is deeper than the root, which is obtuse 

 and truncate. The enamelled summit of 

 the crown is granulate at the margin, and 

 raised in the middle into an obtuse emi- 

 nence, disposed in large transverse, parallel, 

 sometimes wavy and rather sharp ridges. 

 With teeth of this form are sometimes found 

 others of smaller size, with more convex 

 rounded crowns, doubtless forming the ex- 

 tremes of the multiserial pavement which, 

 Fi 31 as in modern sharks and rays, covered 



Ptychodu* latissimus. the broad jaws of the Ptychodonts. Large 

 dorsal spines have been found so associa- 

 ted with the above-described teeth as to indicate the affinity 

 of the Ptijclwdus to the Cestraciont family of sharks. All the 

 specimens and species referable to this genus have been found 

 in the cretaceous strata. 



Family II. — Squalid^. 

 {Sharks.) 



The well-marked, saw-shaped tooth (fig. 32), so closely re- 

 sembles the lower jaw-teeth of the sharks, called " grisets " b}- 

 the French (Notidanus, Cuv.), as to be referred to that genus 

 by Agassiz. Such teeth nevertheless occur in strata of oolitic 

 age (Notidanus Miinsteri, Ag., fig. 32). Other species — e. g., 

 N. pcctinatus — are found in the chalk of Kent ; and N. serra- 

 tissimus, in the eocene clay at Sheppy. 



The tooth (fig. 33) on which Agassiz has founded the genus 

 Corax, indicates by its close resemblance to those of Carcharias, 

 its relationship with the true sharks (Squalidcc). Most of the 



