305] G. E. Dorsey 107 



THE HABITAT OF BELEMNITELLA AMERICANA AND 

 MUCRONATA 



By GEO. EDWIN DORSET 



The question as to whether or not the almost cosmopolitan 

 range of certain type fossils is an indication of similar life 

 conditions over wide areas, or to what extent it may indicate 

 especially hardy, easily or rapidly adaptable organisms, has 

 never been tested. Presumably in the case of some organ- 

 isms cosmopolitanism is attained because of the wide extent 

 of favorable environments, while in the case of other organ- 

 isms they are less affected by the environment or more 

 adaptable to it. 



With the idea of ascertaining whether similar conditions 

 of deposition of the fossiliferous sediments as evidenced by 

 identical or similar lithology can be correlated with the 

 occurrence of particular species of wide-ranging fossils, I 

 have taken the form Belemnitella americana with its Euro- 

 pean analogue, Belemnitella mucronata, and have searched 

 the literature .with regard to their occurrence and the char- 

 acter of the sediments in which they are found. The result 

 is very interesting and fairly conclusive. 



To anticipate these results I have found that in almost 

 every instance where these types are found, they are associ- 

 ated with a lithology which indicates practically identical 

 conditions of deposition. On the other hand, and as a corol- 

 lary to this fact, they show no evidence of adaptation. These 

 species appear with unusual suddenness and abundance to- 

 ward the top of the Tipper Cretaceous, spread rapidly and 

 to great distances, and die out before the dawn of the Ter- 

 tiary as abruptly as they appeared. Throughout this com- 

 paratively short period geologically they apparently main- 

 tain a rigidly uncompromising individuality. However, the 

 fact that the fossil form called Belemnitella is merely a small 

 internal vestige of a once-enveloping shell, and hence far 



