l8 SMITHSONIAN INSTITUTION 



the weathered surfaces and their precise position in the geological 

 section could not he accurately determined. 



Thirty-eight turtle specimens representing several genera and 

 species and varying in size from those with shells a few inches in 

 length to giant tortoises nearly a yard long are also included in the 

 collection. In two instances complete skulls and other hones of the 

 skeleton were found associated with the carapace and plastron. Turtle 

 skulls are extremely rare, and both of those obtained appear to be 

 perfect with the lower jaws in place. Perhaps in no other formation 

 of North America is there such an abundance of turtle specimens 

 as are found in the Bridger. They occur almost everywhere in the 

 formation both horizontally and vertically. In one locality we found 

 an outcropping layer 50 feet or more in length that was composed 

 almost exclusively of turtle shells. These were packed together so 

 closely that it was impossible to remove one specimen from the mass 

 without damaging a number of others. So far as could be determined 

 in the field all appeared to pertain to a single genus. 



Although there can never be more than surmise as to the catas- 

 trophe that brought about this great destruction of chelonian life, 

 several possibilities might be mentioned. A shower of volcanic ashes, 

 noxious gases, or a sudden Mow of superheated water — any one of 

 these would be capable of destroying these animals. That volcanoes 

 were active in Bridger times is abundantly proved by the well estab- 

 lished fact that many of the layers of this formation are composed 

 exclusively of volcanic ash. 



The concentrated accumulation of so many turtles in this one spot 

 may possibly be explained as having been brought about by current 

 action. A flowing stream during a freshet may have assembled the 

 shells from a considerable area and floating down stream these were 

 caught in an eddy or were stranded on a sandbar and thus were 

 brought together in this one spot. The many broken shells and the 

 manner of their overlapping, one shell upon another, would make 

 such an explanation plausible. 



Despite their abundance, however, well preserved turtle specimens, 

 while not rare, are found only occasionally. Crocodiles are also com- 

 mon and in the collection made there are no less than nine skulls, two 

 of which are associated with much of their skeletons. A considerable 

 variety of lizards is known from the Bridger and of these small 

 reptiles several specimens of a fragmentary nature were collected. 



The value of the collection was greatly increased through the coop- 

 eration of Dr. W. H. Bradley of the United States Geological Sur- 

 vey, who secured the necessary field data from which he will prepare 



