^ 



PRIMORDIAL PERIOD. - 37 



* 



a mucli larger size than our shell, and the small pits that similarly mark 

 its surface are six-sided, instead of four-sided as in ours. The surface of 

 T. siluriana Davidson, another allied species, has the pits an-anged in radiat- 

 ing instead of oblique lines. 



Position and locality. — Shales of the Primordial period and probably of 

 the Potsdam epoch, Pioche, Nevada. 



MOLLUSCA VERA. 



Class GASTEROPODA. 



Subclass PTEROPODA. 



Order THECOSOMATA. 



Family HYOLITHIDiE. 



Gbntts HYOLITHES Eicliwakl, 1840. 

 Hyolithes primordialis Hall(?). 

 Plate I, fig. 5 a, b, c, d, aud e. 

 Theca primordialis 'H.al], 1861, Geol. Surv. Wisconsin (p.imph.), 48. 

 Theca primordialis Hall, 1862, Geol. Surv. Wisconsin, i, 21. 

 Theca primordialis Hall, 1863, Sixteenth Ann. B,eg. Rep. N. Y. State Cab., 135. 

 Hyolithes primordialis Hall and Whitlield, 1873, Twenty-third Ann. Reg. Rep. N. Y. 



State Cab., 242. 

 Hyolithes primordialis ?, White, 1874, Geog. & Geol. Exp. & Surv. west 100th Merid., 

 Prelim. Rep. Invert. Eoss., 6. 



Shell slender, acutely pointed; test rather thin; transverse section sub- 

 semicircular or subtrihedral ; edges or longitudinal angles neai'ly or quite 

 straight, meeting at an apical angle, varying in different specimens from fif- 

 teen to twenty degrees ; dorsal side slightly convex or nearly flat along the 

 middle ; ventral side forming a nearly semicircular arch in transverse sec- 

 tion, but it sometimes shows an obtuse, rounded angle, extending from apex 

 to aperture along the median line ; the lateral angles, formed by the junc- 

 tion of the dorsal and ventral sides, sharply rounded ; margin of the aper- 

 ture at the flattend side extended and broadly rounded ; that of the convex 

 side nearly transverse, but sometimes showing a slight emargination at the 

 middle. Upon the flattened or dorsal side of the solid stony core of one of 

 the specimens, from which the test is removed, there appears a very slightly 

 raised longitudinal ndge, the width of which at all parts of its length is 



