342 AN INTRODUCTION TO THE STUDY OF FOSSILS 



otomi, likewise extinct, best known in the world-wide genus 

 Pleuracanthus ; c. Acanthodii, a large Paleozoic group, exempli- 

 fied by Acanthodes, Lower Devonian to Permian ; d. Selachii, 

 upper Paleozoic to present. Examples of the selachians are : 

 (i) Cestracion, found from the Jurassic to the present, most 



Fig. 145. — The extinct marine shark, Carcharodon megalodon Charlesworth, from 

 the Miocene (Calvert) of Maryland. Natural size. It is much more abundant in 

 the Eocene of South Carolina. A, anterior tooth, inner face. B, same in profile. 

 (From Eastman.) 



characteristic of the Mesozoic. This genus shows close rela- 

 tionship to some Pennsylvanian and Permian forms, and sur- 

 vives to-day in the two or three species of Port Jackson shark 

 of Australia, Japan and California. (2) Lamna and (3) Car- 

 charodon (Fig. 145) (both from Cretaceous to present) have 

 teeth with a sharp, wedge-shaped crown fixed upon a some- 

 what bifurcate base which is much compressed antero-poste- 



