MANSFIELD: MESOZOIC OF FORT HALL RESERVATION 39 



bratuloids, and Myaphoria lineata(?), project from the weathered 

 surfaces. 



The formation is fairly resistant to erosion and forms low, broad 

 ridges and sloping interfluvial areas. Its thickness is estimated at 

 about 1,500 feet, although there is some uncertainty because of 

 complexities of structure. 



Numerous collections were made from this formation by G. H. 

 Girty, who furnishes the following discussion of the fauna: 



The Portneuf fauna is the most varied and interesting of the three 

 Triassic faunas of the Fort Hall Reservation. Echinoid spines occur in 

 a number of locations but they are not plentiful. On the other hand, 

 segments of the stems of Pentacrinus are often found in great abundance. 

 In two localities bryozoa are abundant, small branching types, super- 

 ficially resembling the Carboniferous genus Batostomella. Several new 

 genera and species are indicated by thin sections. Brachiopods are 

 abundant, but confined to two families. This is the horizon of Terebra- 

 tula semisimplex, but there are also several other terebratuloid types 

 which are apparently undescribed. An undescribed species of Spiri- 

 ferina occurs in many of the collections, and there may be a second 

 species. 



Pelecypod types are so numerous, and at the same time so poorly pre- 

 served in many cases, that it would be inexpedient to do more than men- 

 tion the most interesting. No species is more frequently met in this 

 fauna than one which was figured by Meek as Myaphoria lineatat. 

 The locality of Meek's specimen is given as Weber Canyon, and the 

 horizon as Jurassic. I can not but think that there is some mistake in 

 the stratigraphic position of his material, which was said to be above 

 the quarry rock, the quarry, I assume, being then as now in the Nugget 

 sandstone. Compared with their abundance in the two lower forma- 

 tions, Pectens are scarce in the Portneuf. A large form with very 

 coarse ribs occurs in several collections, and there are other species, 

 both large and small. A large Pteria or Bakewellia has been found at 

 many localities; also a Myalina or Mytilus. Leda is present, and 

 Nucula, together with types suggesting Pinna, Myacites, Pleurophorus, 

 Astarte, Cuculaea, and other forms. One locality has furnished a few 

 specimens of Ostrea, not only a plicated form similar to that of the 

 Fort Hall formation, but also a smooth type. 



The scaphopods too are represented in this fauna by one or two 

 species of Dentalium. 



Gastropods are less abundant than pelecypods, the only common type 

 being a small Natica, probably N. lelia. Several small species of Pleuro- 

 tomaria? have been collected, and also shells suggesting the genera 

 Holopea, Nerita, and Macrocheilina. The most interesting representa- 

 tive of this type, however, is a beautiful little species apparently belong- 

 ing to the Carboniferous genus Schizostoma, or at all events to the 

 euomphaloid group. 



