36 MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 



Meekoceras zone, 6 have not been recognized here, but there is 

 evidence of more than one Ammonite horizon. 



Above the Meekoceras zone for about 800 feet are massively- 

 bedded and thin-bedded gray to brown limestones containing 

 large numbers of small brachiopods, chiefly Pugnax and tere- 

 bratuloids, and pelecypods, Myalina and others, with inter- 

 vening calcareous shales. The lithology of the shales and 

 thinner-bedded limestones is much like that of the Woodside. 

 The limestones weather with a sort of velvety appearance and 

 are very fossil if erous. The presence of the small brachiopods in 

 the massive limestones near the base is a convenient guide to 

 the Ross limestone where the Meekoceras zone is not available. 



The upper part of the Ross limestone for about 500 feet con- 

 sists of a dense calcareous shale, gray to olive-greenish in color 

 and weathering brown to yellow. These shales form conspicuous 

 cliffs and are mainly non-fossiliferous. 



G. H. Girty contributes the following faunal discussion of the 

 Ross limestone: 



The fauna of the Ross limestone consists chiefly of brachiopods, 

 pelecypods, and cephalopods. The brachiopods and cephalopods are 

 largely restricted to zones which are narrow and possibly of small ex- 

 tent, but where found at all they are abundant. The brachiopods 

 comprise a Lingula, a Terebratula, and a Rhynchonella, those terms 

 being employed in a broad and general sense. The Rhynchonella 

 closely resembles the Carboniferous species Pugnax utah and, as the 

 Triassic occurs in the general region from which the type specimen 

 was obtained, typical Pugnax utah may indeed be the Triassic form, 

 as was suggested to me several years ago by Mr. Breger. A few 

 specimens of a small Discina have also been collected. 



The pelecypods consist mostly of pectinoids of which there are many 

 species. They probably include representatives of both the Pectinidae 

 and Limidae and they occur in some places in vast numbers either alone 

 or associated with other forms. Like most of these Triassic fossils, they 

 belong to undescribed species, though one form can probably be referred 

 to Aviculipecten thaynesianus. Other types of pelecypods are much less 

 common. The one most frequently found is that described by White 

 as Volsella platynota, but if my specimens really belong to White's 

 species I believe that it is a Myalina. A small alate shell, which may 

 belong to Bakewellia or Pteria, has been found, and also forms which 

 suggest the genera Schizodus, Cardiomorpha, and Pleurophorus. 



6 Smith, J. P. The distribution of Lower Triassic faunas. Jour. Geol., 20: 17. 

 1912. 



