MANSFIELD : MESOZOIC OF FORT HALL RESERVATION 37 



These last are so poorly preserved that their generic relations, even as 

 based on external characters, are conjectural. 



The cephalopods have been carefully investigated to the almost com- 

 plete neglect of the rest of the Triassic fauna of this region. The Ross 

 limestone is the horizon of the cephalopods par excellence, the Meeko- 

 ceras zone. Nevertheless, the collections studied, which were not made 

 with special reference to any one group of organisms, contain neither 

 very numerous nor very complete specimens. The following species 

 have been identified with more or less certainty: Meekoceras much- 

 bachanum, Meekoceras gracilitatis, Paranannites aspenensis, Ophiceras 

 dieneri, Flemingites russelli, Clypites tenuis. 



Gastropods are so rare in the Ross limestone that they might with 

 little loss be neglected in a hasty survey of its fauna. One collection 

 contains an abundance of small naticoid shells (Natica lelial), but of 

 much more interest is the occurrence in another collection of a species 

 of Bellerophon. There can hardly be a doubt of the generic relationship^ 

 of this form which resembles the Pennsylvanian species B. crassus. The 

 Bellerophontidae, though profusely developed in the Paleozoic and al- 

 most confined to that era, have been known in other parts of the world 

 to range also up into the Mesozoic. 



Fort Hall formation. The Fort Hall formation is named from 

 old Fort Hall, the site of which is in the valley of Lincoln Creek, 

 which appears on some maps as Fort Hall Creek. The formation 

 occupies a prominent ridge along the north side of this valley. 

 The rocks lie conformably on the Ross limestone. The dividing 

 line is drawn on both lithologic and faunal grounds. There are 

 four fairly well defined subdivisions. 



(1) The base of the formation is a soft and somewhat sugary, 

 yellow calcareous sandstone about 50 ± feet thick, sparingly 

 fossiliferous and containing at one locality a bed of yellowish 

 sandy limestone about 15 feet thick, with plicated oyster-like 

 pelecypods, terebratuloids, and other forms. This bed is overlain 

 by white calcareous sandstone weathering red or pink. 



(2) Above these sandstones there is a gray or yellowish, sili- 

 ceous, dense limestone containing large pectin oids and irregular 

 cherty nodules and streaks that weather with a rough surface 

 and project along the bedding planes. This limestone forms 

 rough ledges and high points. The thickness of this series is 

 estimated at 100 ± feet. 



(3) Above (2) and observed at only two localities, sees. 36 and 

 26, T. 3 S., R. 37 E., Boise M., is a set of sandy and shaly gray 



