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



plexing as any in the whole range of palEsontological science. The most 

 profitable method of investigating fossil remains is by comparison with existing 

 forms, and it is fortunate that all the genera are not yet extinct ; and though the 

 fossil forms far outnumber the existing ones, there are still examples of Carcharo- 

 don, Lamna, Oxyrhina, and Odontaspis. As already observed the characters of 

 the first three existing genera as defined by Miiller and Henle, having reference 

 to the size, form, and position of the tins and tail, are identical, and so far as 

 those tests are concerned do not indicate any generic differences. Odontaspis, on 

 the other hand, differs from those named, in the form and position of the fins and 

 tail, to such an extent as to induce Sauvage to place it as a separate family ; 

 whilst, judging from the teeth alone, Agassiz and others have regarded them as 

 at most a sub-genus of Lamna. After carefully considering the divisions of the 

 Lamnidse attempted by Miiller and Henle, Agassiz expresses the opinion that 

 they may be tangible enough, and very serviceable in existing forms, when the 

 whole of the structure of the fish may be studied, but of little use when consider- 

 ing and attempting to decipher a mass of detached fragments, in nearly all cases 

 consisting of isolated teeth or vertebrae. 



The Cretaceous fish-remains, Agassiz says, are characterized by a large 

 number of new types which have not existed at an earlier period. The group of 

 teeth with crenulated margins appears for the first time ; and amongst the smooth 

 teeth are several types equally new, such as Otodus, Oxyrhina, and the subulate 

 Lamna, or Odontaspis. The greatest difficulty consists in distinguishing between 

 Otodus and Oxyrhina ; and between Otodus and certain forms of Lamna ; also, it 

 is necessary in certain cases to renounce the hope of rigorously determining 

 fragments of teeth deprived of their roots. The difficulties which encompassed 

 Agassiz have increased since he wrote ; and the discovery of numerous forms in all 

 parts of the world, and the accumulation of large collections of fish-remains in 

 public and private museums, have only served to produce a still more comj)licated 

 result and render still more difficult a satisfactory system of classification. 



The existing Lamnidse as represented in Carcharodon and Oxyrhina indicate 

 two distinct forms of dentition both free from lateral denticles, the former with 

 serrated anterior and posterior margins, and the latter with those margins devoid 

 of serrations. These are abundant in the Tertiary strata, and Oxyrhina also in 

 the Cretaceous. 



Agassiz held the opinion that the serrated margin of the teeth was of generic 

 importance, and this led him to include a number of forms with well-developed 

 lateral denticles in the genus Carcharodon because they had serrated margins. 

 The question immediately arises whether the serrated margin, or the presence of 

 secondary cones is of greater generic importance. So far as the evidence of the 

 existing species goes, there is no trace of lateral cones or denticles, and if 



