28 SHARKS. 



characters of great value in the determination of both recent and 

 extinct forms. 



The teeth of the genus Lamna present an elongated triangular 

 sub-compressed form, with the anterior or outer surface less convex 

 than the posterior, especially in the median teeth and in those of the 

 lower jaw. The margins are trenchant and converge to a sharp 

 point, equally or symmetrically in the median teeth, but unequally 

 in the lateral ones in which the point is inclined backwards. In 

 some species, as in the porbeagle shark {Lamna Cornubica), and in 

 that great extinct species of which the teeth occur in the London 

 clay and other members of the eocene formation in this country, and 

 on which Agassiz has founded his sub-genus Otodus,{l) the teeth are 

 complicated by a small accessory cusp on each side of the base of 

 the principal cone. In other species, as in the Lamna oxyrhina of 

 Cuv. and Val., (PI. 3, fig. 1,) the accessory cusps are wanting, at least 

 in large and full grown specimens : in both subgenera of Lamna, the 

 third vertical row of teeth counting backwards in the upper jaw is 

 much smaller than the contiguous rows(2) ; the rest gradually diminish 

 in size as they are situated nearer the angles of the mouth. In the 

 sub-genus Odontaspis, the teeth are narrower than in Lamna, the mid- 

 dle cusp is longer, straighter, and more acute, and is provided with 

 two similar sharp pointed cusps on each side of its base. In the 

 upper jaw, the fourth, fifth, sixth, and seventh vertical rows of teeth 

 are smaller than the contiguous ones ; in both jaws, the first tooth is 

 small and the posterior teeth progressively diminish as they approach the 

 angles of the mouth. (3) The size and strength of the teeth thus modi- 

 fied for piercing and lacerating render the Odontaspis the most formi- 

 dable among the shark tribe ; and the habits of the typical species, 

 {Odontaspis ferox) as indicated by its trivial name, correspond with 



(1) PL 5, fig. 5. 



(2) M. Agassiz in his description of the dentition of this genus, mentions as one peculiarity 

 of it that, " la troisieme et quelquefois la quatrieme et la cinquieme dent de la mdchoire inferieure 

 est sensiblement plus petite que les autres, tandis qu'a la mdchoire sup^rieure les dents, a I'ex- 

 ception de la premiere qui est plus; petite que les suivantes, vont en diminuant uniformement 

 de grandeur jusqu'^ la partie posterieure de la gueule." In a skuU of the Lamna Cornubica now 

 before me, I find, as in the Lamna oxyrhina, (PI. 3, fig. 1,) that the sudden diminution of size in 

 the third tooth is characteristic of the upper and not of the lower jaw. 



(3) PI. 3, fig, 2. 



