318 BRAXSOX AXD GREGER AMSDEX FORMATIOX OF AVYOMIXG 



Pedicle valve strongly convex, with greatest convexity opposite the 

 hinge line ; beak strongly incurved ; cardinal area high, short, height and 

 breadth as 1 to 3. Cardinal area not sharply defined, bnt with the shell 

 rounding to meet the area; lateral slopes of the valve convex, marked by 

 9 to 11 subangular to rounded plications. The mesial sinus is broad and 

 shallow and originates at the beak; a median pUcation starts at the um- 

 bone and increases in size backward; two plications come into the sinus 

 by bifurcation of the marginal plications, and these may remain small or 

 become subequal to the median plication. 



Brachial valve much less convex than the pedicle. The mesial fold 

 which originates at the beak is set off from the rest of the shell by grooves 

 that are deeper and wider than those between the plications. Near the 

 beak the fold is not elevated above the rest of the shell, but anteriorly it 

 becomes prominent. A median furrow originates at or near the beak, and 

 two shallower, lateral furrows originate about the middle of the fold. 



Remai'k^. — In our earlier work on the Amsden we included this form 

 with S. pellccnsis, but we now give it species rank on account of the con- 

 stantly short hinge line and high and ]iarrow area. It is the most abun- 

 dant Spirifer in the Amsden. 



SPIRIFER SHOSHOKEXSIS n. sp. 

 Plate 18, fibres 26 and 27 



Shell of medimn size ; wider than long ; greatest breadth at hinge line. 

 Width of largest specimen, 50 millimeters ; of smallest specimen, 29 milli- 

 meters; length of largest specimen, 32 millimeters; of smallest, 12 milli- 

 meters. Thickness of pedicle valve of largest specimen, 12 millimeters. 



Pedicle valve strongly convex ; greatest convexity a little in front of 

 the hinge line; beak strongly curved and projecting about one-fourth 

 of the length of the valve beyond the hinge line. Lateral slopes of the 

 pedicle valve convex, each marked by about 17 rounded plications. The 

 first eight lateral plications on each slope are formed by the bifurcation 

 of plications near the back of the valve. The plications outside of the 

 first eight do not bifurcate. The mesial sinus is shallow and poorly de- 

 fined. A median plication starts at the umbone, increases slightly in size 

 forward, and in some specimens bifurcates. Two plications come into the 

 sinus near the umbone by bifurcation of the marginal plications and be- 

 come subequal in size to the median plication. 



Brachial valve much less convex than the pedicle. Greatest convexity 

 near the middle of the valve, with lateral slopes and front slope subequal. 

 Each lateral slope is marked ])y about 15 subangular to rounded plica- 



