Davis — On the Fossil Fish of the Cretaceous Formations of Scandinavia. 373 



C. rondeletii be taken as the type, then it becomes anomalous to include in the 

 genus such forms as Carcharodon heterodon, Ag., C. megahtis, Ag., C. auriculatus, 

 Ag., C. angustidens, Ag., and others. In form and size they approach very nearly 

 to some of the larger species of Otodus, such as 0. obliquus, Ag., and if the margins 

 of the latter were serrated would be indistinguishable from it. 



Noetling has already suggested that Otodus should be joined to Carcharodon ; 

 but it appears much more reasonable, either that the forms associated with 

 Carcharodon which possess lateral denticles should be considered as species of 

 Otodus, or regarded as a separate genus. 



Having regard to the remaining genera Lamna, Otodus, and Odontaspis, 

 existing species of the first and last still survive ; but unfortunately, hitherto, no 

 specimen of a living Otodus has been discovered, but so many wonderful types 

 have been found by deep sea dredging and more careful search, during the past 

 few years, that it may not be impossible that still others may be brought to light. 

 At any rate, until more reliable information is accessible, it may best serve the 

 purpose of the palaeontologist to regard the dismembered fragments simply as 

 "forms" exhibiting certain tendencies of a more or less definite character. 

 Sufficient has already been advanced to show that any lines of absolute demarca- 

 tion into genera, it might almost be said species, is impossible ; and the researches 

 of every fresh student may lead to new opinions formulated in new varieties of 

 nomenclature. Already the subject is almost hopelessly confounded ; the trans- 

 position of species is bewildering ; and after all there is no firm basis on which to 

 build up a natural classification. 



One of the principal difficulties appears to be, that it should be desired to make 

 an extremely large series of fossils, representing an enormous development of the 

 Selachians, fit to a minimized series of living representatives which are 

 rapidly dying out; and that sufficient credit is not given to the variety and 

 number of the species which obtained during the ascendancy of the family. 

 Accepting this view it may be advisable to regard the teeth, as already suggested, 

 as "forms" representing members of the family, and classify them accordingly; 

 and it scarcely seems necessary to suggest that the successive redistribution of 

 species amongst existing, or newly devised genera, is to be deprecated. The 

 genera as defined by Agassiz embrace already a wide range of species, and have, 

 hitherto, proved adequate. They are universally known and accepted, and have 

 tolerably well-defined limits. Taking as types, Otodus obliquus, and the existing 

 species Lamna cornubica, and Odontaspis ferox, the palaeontologist will be able to 

 group the ever- varying fossil forms around these centres, and though they may 

 possess characters expressing relationship with more than one species, succeeding 

 discoveries may show that these only express the connecting links of an unbroken 

 line of evolutional development. 



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