SINCLAIR— THE " TURTLE-OREODON LAYER." 461 



algal crust ; none were seen included in it. These algae grew at the 

 edge and partly within a shallow stream cutting the " turtle-oreodon 

 layer ". Elsewhere throughout Indian Creek basin algal crusts and 

 balls have been seen at several localities in the upper part of the 

 Titanotherium beds, not far below the Oreodon beds contact, resting 

 on a clay substratum and sometimes growing over bone fragments. 

 Specimens of both types have been sectioned and the material sub- 

 mitted to Dr. Marshall A. Howe, of the New York Botanical Gar- 

 den. His studies thereon have been most seriously handicapped by 

 recrystallization of the organically formed limestone, almost com- 

 pletely obliterating the original cellular structure. Dr. Howe thinks 

 that one organism dominates, and that there is no very serious ad- 

 mixture, and, furthermore, that " it is most reasonable to suppose 

 that the plant belongs with the Myxophycese (Cyanophycese or Blue- 

 green Algae), and under this class its family is most likely to be the 

 Rivulariaceas, though one can not feel absolutely confident of it. 

 One would not be justified in referring it to any known genus, living 

 or fossil, and there is nothing very definite, concrete, or detailed on 

 which to establish a new genus." 



In formulating a theory of origin for the " turtle-oreodon layer " 

 conditions controlling the deposition and preservation of the nu- 

 merous fossils, as well as lithologic and stratigraphic data, must be 

 considered. Every collector is at once struck by the enormous num- 

 bers of fossil turtles of all sizes. These are not aquatic forms, but, 

 as Dr. Hay^ has pointed out, are closely related to our living land 

 tortoises, so far as can be determined from the material available, 

 mostly empty carapaces. These, quite irrespective of size, do not 

 lie in death pose, but may be upside down or on edge. Heads and 

 limbs are missing and have either been eaten by carnivores or de- 

 cayed and dropped off when the shells were moved by the trans- 

 porting agent. The majority of the other fossils found are mam- 

 malian skulls, often with lower jaws attached, and lie in all sorts of 

 positions. Loosely inserted teeth are sometimes missing, but there 

 is little or no indication of abrasion by movement in material found 

 outside the sandstone channels. Many of the skulls were reduced 



2 O. P. Hay, " The Fossil Turtles of North America," Carnegie Institu- 

 tion of Washington. Publication No. 75, p. 385, 1908. 



