246 The American Geologise. April, 1896 
Fame species a series of teeth which present a considerable range <if va- 
riation among themselves. This favorable circumstance, in the author's 
opinion, affords the key to an understanding of the dentition of the sev- 
eral forms represented; and the specimens are accordingly described as 
pertaining to definite regions of the mouth, from analogy with recent 
forms. 
Four of the seven genera that are described belong to the family 
Lamnidce, one of them, Hypotodus, being new; the remainder are frag- 
ments of Carcharias, Notidanus and Myliobatis. In the descriptions 
of Carcharodon one can but note the contrast between what may be 
called the German method and that followed by the English and other 
schools, namely, the tendency to isolate and individualize minor dis- 
tinctions and to split up larger groups into a number of smaller ones. 
For instance, Dr. Jaekel retains the genus Otodus in its primitive sense, 
the species of which are now generally distributed among Lamna, < 'ar- 
charodon and Oxyrhina; he recognizes three species of the last named 
genus, whereas most authors admit only two: and he holds that the 
majority of Carcharodon species described by the older authors are dis- 
tinct, while A. S. Woodward, in his "Catalogue of Fossil Fishes," in- 
cludes no less than eleven of these under the synonymy of C. auricula 
his (Blv.). 
But in this paper the author exceeds even Agassiz in his tendency 
toward minute differentiation. Agassiz's Odontaspis verticalis, from 
the London clay of Sheppy, which has always been regarded as a 
"good " species of Odontaspis, now finds itself elevated to be the type 
of a new genus, Hypotodus, and a new Russian species (H. trigonal is) 
is placed alongside of it. Moreover, owing to the author's inability to 
bring two of the Russian specimens of Carcharodon into perfect agree- 
ment with any one of Agassiz's figures, he proposes the new specific 
title of C. sokolowi for a form which may be readily associated with 
C. auriculatus. The most extreme position of all, however, is taken by 
Dr. Jaekel in regard to the genus Oxyrhina, which he states to be 
" traceable with certainty only as far back as the Eocene;" some of 
the Cretaceous forms that are usually placed here, being, in his opinion, 
unquestionably referable to other genera. 
It is doubtful whether the majority of paleichthyologists will agree 
with all of these conclusions. In regard to the last mentioned, it need 
only be pointed out that Oxyrhina is as typically a Cretaceous genus as 
is Otodus auct., which Jaekel still upholds ; the only noteworthy dis- 
tinction between them consisting in the presence or absence of lateral 
denticles. Not only have Lawley's researches made known the nearly 
complete dentition of certain Tertiary species of Oxyrhina, but its Cre- 
taceous representatives, described by Bassani and the writer, furnish 
the most perfectly preserved individuals known among the Lamnidai. 
Except for the reduction in size, the dentition of Oxyrhina has remained 
practically unaltered since its initiation in the Cretaceous ; the allied 
genus Carcharodon has also suffered a diminution in size, but it is 
reasonable to suppose its dentition has not been otherwise affected. 
