287] B. Wade 89 



Myrtaea, Phacoides, Docinia, Tellina, Solyma, etc., are absent 

 from the present McNairy County collection though it is 

 probable that further collecting may reveal some of them. 



The Scaphopoda are represented by the two families Den- 

 taliidae and Siphonodentaliidae. The former originated in 

 the Ordoviciari and are abundantly developed in the Creta- 

 ceous and Tertiary. They are represented at Coon Creek by 

 one genus and two species, one of which is very common. 

 The family Siphonodentaliidae is first found in the Creta- 

 ceous. At Coon Creek it is abundantly represented by the 

 minute form Cadulus obnutus (Conrad). 



The Gastropoda are the most interesting class in the Coon 

 Creek fauna. It will be noted from the list given above that 

 the number of genera and species of the Gastropoda is con- 

 siderably greater than the number of the Pelecypoda, yet, 

 probably in every cubic yard of the Coon Creek sediments 

 the number of bivalve individuals exceeds the univalve indi- 

 viduals several times. In all the faunas previously reported 

 from the Cretaceous of Eastern United States the bivalve 

 species are more numerous than the univalve species. This 

 majority among the latter may be due simply to the fact that 

 in the Upper Cretaceous seas of those regions the pelecypods 

 predominated in number of species as well as individuals, or 

 it may be that a greater number of gastropod species existed 

 in all the Cretaceous seas but were not preserved sufficiently 

 to be recovered from the sediments. The chance of preser- 

 vation of gastropod shells is not as good as it is for pelecy- 

 pods, first, because the number of individuals per species of 

 the Gastropoda is rarely ever as great as it is among the 

 Pelecypoda. Second, the essential constituent of univalve 

 shells is aragonite, and this mineral is much less stable than 

 calcite, which is the essential constituent of the majority of 

 pelecypod forms. Third, a gastropod shell is in greater 

 danger of being crushed by the pressure of the inclosing 

 sediments because of lack of support from within the shell. 

 The body cavity of gaping bivalved shells is almost of neces- 

 sity filled, while the sediments are intruded less readily 



