303 



noticed the specimen from the English chalk. My comparisons had been 

 made with the figures of Agassiz, Dixon, and Reuss, and in none of these do 

 the teeth exhibit so conspicuous a lateral extension of the base of the crown 

 as in the American specimens and the English chalk specimen of our Museum. 

 An exception to this statement may be made in reference to Fig. 26, Plate 

 XXX, in Dixon's Geology of Sussex, representing a tooth, which is referred 

 to La?nna acuminata. 



Measurements of the specimens referred to Oxyrhina extenta are as follows: 



Specimens represented in Plate XVIII. 



Fig. 21. 



Fig. 22. 



Fi 2 . 23. 



Fis. 24. 



Fig. 25. 



Lines. 



Length from notch, of root . 

 Length of crown at middle 

 Breadth of crown at base 



Breadth of root 



Thickness of root 



9 



12 



5 



Lines. 



12 



8 



15 



17 



Lines. 

 8£ 



12 

 13 

 4£ 



Lines. 

 14 

 12 

 16J 

 18 

 5 



Lines. 

 10 



»£ 

 13 

 14 



4 



LAMNA s. OXYRHINA. 



The teeth of Lamna are in general characterized by the long, narrow 

 crown, with a single denticle on each side of the base, and a strong root with 

 narrow branches separated by a deep notch Those of Oxyrhina usually 

 have a broader crown without lateral denticles, and also have a broader root 

 with a 'shallower notch. In both genera, however, the proportion of breadth 

 to length, and most other characters, except the presence or absence of the 

 lateral denticles, vary in different parts of the jaws. In both, the side teeth 

 are wider than those in advance, the disproportion usually being greater in 

 Oxyrhina than in Lamna. Some of the teeth in the two genera so nearly 

 assume the form of one another, that when isolated fossil teeth of either are 

 found without the base it is sometimes difficult to know to which to refer 

 them. 



A number of times I have seen specimens of teeth, reputed to have been 

 derived from the Cretaceous formation, which so closely resemble those of 

 certain Tertiary species of Lamna, except in the possession of lateral denti- 

 cles, that I have suspiciously regarded them as pertaining to the same. The 

 absence of denticles I thought might be accidental or abnormal. The report 

 that the teeth had been found in a Cretaceous formation I suspected might 



