Davis— 0« the Fossil Fish of the Cretaceous Formations of Scandinavia. 367 



Corax is known only by the teeth. They are sufficiently distinctive in form 

 and of world-wide distribution. They are distinguished from the teeth of the 

 existing Carcharias and Galeus, to which they bear considerable resemblance, by 

 having no central cavity. The teeth of the existing fishes are hollow. 



Teeth of Otodus are only known in a fossil state. The genus was instituted 

 by Agassiz for teeth which exhibited forms intermediate between Carcharodon on 

 the one side, and Oxyrhina and Lamna on the other. They agree with the 

 genus Lamna in possessing lateral denticles; but whilst those of Lamna and 

 Odontaspis are cylindrical and sharply pointed, those of Otodus are larger, flat, 

 and blunt. The same characters may also be said to distinguish the crown of 

 each respectively. The root of Otodus is very large and thick ; but it has not the 

 extended horn-like projections which characterize Lamna.* From Oxyrhina this 

 genus is distinguished by the presence of the lateral denticles, and from Carcha- 

 rodon it is separated by the absence of the serrated margins which characterize 

 the former. Agassiz regards this character as of great importance, and though it 

 may be of doubtful value in some species placed on the confines of the genera, he 

 regards it as not less decisive in the greater number. He says, however (p. 266) : 

 "II en est de ceci comme de toutes nos diagnoses ; elles ne sont vraies que dans 

 certaines limites, et c'est a la perspicacity du naturaliste k reconnaitre et k, sentir 

 oil ces limites se trouvent." The microscopical structure of the teeth is solid and 

 massive, as in Carcharodon and the true Lamna. The genus appears first in the 

 Cretaceous rocks, was abundant in the Tertiaries, and died out before the existing 

 period. 



The genus Oxyrhina includes one or two existing species. The type of the 

 genus, and the one best known, is Oxyrhina spallamani {Lamna oxyrhina, Cuv. 

 and Val.). The teeth are completely free from lateral denticles, and the margins 

 are smooth. The crown of the tooth is very similar in form to that of Otodus; 

 and imperfectly -preserved specimens of Otodus, from which the base has been 

 broken so as to detach the lateral denticles, can with difficulty be distinguished 

 from Oxyrhina. The fossil teeth attain a considerable size ; they conform 

 generally with the ordinary arrangement of the teeth in sharks ; those situated 

 on the anterior part of the jaws are more lanceolate and acutely pointed than 

 those occupying a median position, whilst the posterior teeth are smaller, 

 triangular, and much compressed. 



The genus Lamna was originally founded by Cuvier,t and embraced a variety 

 of fishes which have since been taken as the types of other genera, amongst them 

 Otodus, Carcharodon, and Oxyrhina, characterized by the form of the teeth. Of 

 the existing sharks, Lamna cornuhica, Cuv., may be taken as the type of the genus. 



* Agass. Poiss. Foss,, vol. iii., p. 266. f Regne Animal, vol. ii., p. 126. 1827. 



