128 Belemnitella Americana and Mwronata [326 



It will readily be observed that such a conclusion is identi- 

 cal with that regarding the conditions under which the chalk 

 is thought to have been laid down, warm,, clear, quiet waters, 

 from 50 to 2000 feet deep, the depth varying with the attitude 

 of the adjacent land masses, currents, etc. The glauconitic 

 and the chalky facies of the Upper Cretaceous may be taken, 

 with a considerable degree of certainty, to indicate practically 

 identical conditions of deposition, conditions which, in view 

 of the comparative rarity of wide-spread similar deposits be- 

 fore and since, were produced by a rather exceptional combi- 

 nation of physical factors. In such surroundings B. americana 

 and B. mucronata appeared and rapidly became extremely nu- 

 merous. Everywhere in the Atlantic province they are found 

 flourishing while warm, quiet, clear waters prevailed. But 

 they rarely strayed far beyond the limits of these surround- 

 ings, and where they did they quickly died out. Finally, the 

 physical changes which brought the Monmouth and Maestrich- 

 tian to a close, in destroying the limited conditions under 

 which the forms could live, caused their complete extinction. 



Nothing dogmatic can be postulated from the study of the 

 habitat of only one or two species. Before any universal con- 

 clusions can legitimately be drawn a very large number of 

 forms must be studied with extreme detail and exactness. 

 The forms I have used may be exceptional in their limita- 

 tions, and therefore possibly not typical examples of guide 

 fossils. But the evidence afforded by the occurrence of Be- 

 lemnitella americana and Belemnitella mucronata undoubtedly 

 points to the conclusion that the wide-spread occurrence of 

 identical forms, which thus constitute intercontinental guide 

 fossils, may be due to the wide-spread occurrence of identical 

 life conditions, rather than to an especially hardy, or easily 

 and rapidly adaptable form. 



BIBLIOGRAPHY 



( 1 ) R. P. WHITFIELD, " Gasteropoda and Cephalopoda of the Rari- 

 tan Clays and Greensand Marls of New Jersey." 1892. 

 U. S. Geological Survey, Monograph 18. 



