90 Upper Cretaceous Fauna from Tennessee [288 



through the aperture in the spiral body cavities of univalves. 

 The shells thus unsupported within become crushed by super- 

 incumbent .sediments and are then rapidly disintegrated. 



In general, the Tertiary and Eecent faunas of North 

 America contain a greater number of univalves species and 

 it may be that about the same proportion existed in all the 

 Cretaceous faunas. Yet it may be that the faunas, as they 

 have been reported, represent the natural proportions in 

 which these animals lived in the Cretaceous sea. It is possible 

 that the gastropods became diversified in the Cretaceous and 

 that this diversification took place only in certain provinces, 

 where environments favored variation. Excavating and ex- 

 tensive collecting in localities where the shells are especially 

 well preserved will probably throw some light on this question. 



In the order Opisthobranchiata there are three families and 

 four genera found in the Coon Creek collection. Of these 

 the family Acteonidae, which had its beginning in the Devo- 

 nian and gained great prominence in the Mesozoic is repre- 

 sented by the genera Act eon and Tornatella. The former in- 

 cludes probably five species and the latter two. The families 

 Ringiculidae and Scaphandridae are each represented by one 

 genus and one species. 



The order Ctenobranchiata is most abundant and is repre- 

 sented by 48 genera, 30 of which belong to the suborder 

 Toxoglossa and 18 -to the suborder Streptodonta. Among 

 the Toxoglossa the family Cancellariidae is first differenti- 

 ated in the Upper Cretaceous. It appears suddenly much 

 diversified in that period and attains its maximum distribu- 

 tion in the late Tertiary and Eecent. This family is repre- 

 sented at Coon Creek by two genera. The most prolific of 

 these is Paladmete, a genus first recognized and described by 

 Dr. Julia A. Gardner who has recently monographed the 

 Upper Cretaceous Mollusca of Maryland. 16 The type spe- 

 cies of Paladmete is very abundant in Maryland and north- 



16 Gardner, J. A., Md. Geol. Survey, Upper Cretaceous vol., Text, 

 p. 412, 1916. 



