368 Davis — On the Fossil Fish of the Cretaceous Formations of Scandinavia. 



Miiller and Henle, amongst other characters of the genus, give:* "the second 

 dorsal and anal small of equal dimensions, opposite ; pit at base of caudal distinct, 

 a keel along each side of the tail." These characters apply equally to Oxyrhina 

 and Carcharodon ; Odontaspis is distinguished by the second dorsal and anal fins 

 being large ; having no pit at the root of the tail, and the absence of keels along 

 the lateral surface. In all three genera the branchial arches are large and situated 

 in front of the pectoral fin. All the characters on which Miiller and Henle 

 based their classification are external, and jDcrtaining to parts of the body which 

 are not usually found in a fossil state, and the palaeontologist, as Agassiz points 

 out,t sees with regret that no account whatever is taken of the skeleton or the 

 dentition ; and it is a remarkable coincidence, that the teeth which present the 

 nearest resemblance, viz. those of Lamna and Odontaspis, should be distinctly 

 removed by the external form of the caudal fin, and by the position and size of the 

 dorsal and anal fins to separate genera; whilst Cetorhinus, Blain. (Selache, Cuv.), 

 Oxyrhina, Agass., and Carcharodon, Miiller and Henle, which have teeth of 

 such great dissimilarity to Lamna, ai'e grouped in the same family in close re- 

 lationship with that genus. 



The teeth of Lamna cornuhica, Cuv., are more or less varied inform indifferent 

 parts of the jaws ; they may be described as possessing a high, median, cone- 

 shaped crown, flat, and compressed antero-posteriorly, with smooth margins ; a 

 single lateral denticle exists on each side of the median cone. The median cone 

 approaches to that of Otodus in form on the one side, but the lateral cones are 

 smaller and more acuminate ; and on the other, it possesses much resemblance to 

 some of the teeth of Odontaspis. It is principally distinguished by the cylindrical, 

 and often twisted form of Odontaspis, which has also longer and more pointed 

 lateral denticles. In Lamna the lateral denticles do not number more than one 

 on each side, but in Odontaspis there are frequently two, and occasionally three 

 on each side. The number is, however, inconstant, and even the teeth of the 

 same fish sometimes are variable. Agassiz very forcibly remarks that the variation 

 observed in the number of the lateral denticles is not calculated to inspii-e a great 

 amount of confidence in their value for determining genera. 



The teeth of Lamna are readily distinguished from those of Oxyrhina, if the 

 base of the tooth and the root are well preserved, because Oxyrhina has no lateral 

 denticles ; from those of Otodus they are less readily distinguished, and some 

 intermediate species appear to bridge over the limits between the two genera, 

 such for example as Otodus appendiculatus^ one of the most common forms, which 

 can scarcely be distinguished from some of the Lamna. 



Some of the characters of Odontaspis have already been enumerated. The 



* Systematische Beschreibung der Plagiostomen, p. 66. ■)■ Poiss. Poss., vol. iii., p. 287. 



