PALAEONTOLOGY OF IOWA. 501 



Spirifer hungerfordi (n. s.). 



PLATE IV. FIG. 1 a - k. 



SHELL inequi valve, semielliptical, transverse or elongated, 

 varying with the age of the animal. Dorsal valve regularly 

 convex, the greatest convexity above the middle, and curv- 

 ing gently on all sides ; beak incurved slightly beyond the 

 hinge line : no defined mesial fold. Ventral valve in the 

 young and half grown shells semielliptical, gibbous above 

 the middle, having twice as great an elevation as the oppo- 

 site valve, the beak much extended above the hinge line 

 and slightly incurved ; hinge line equalling or extending 

 beyond the width of the shell ; sinus shallow, scarcely de- 

 fined above the middle, and producing a slight sinuosity in 

 front : area large and well defined, principally confined to 

 the ventral valve and limited by an obtuse margin, striated 

 vertically ; foramen narrow, acute above, and extending 

 quite to the apex of the shell, the margins or dental lamellae 

 often a little thickened or projecting. 



SURFACE marked by fine simple radii, a few only of which 

 dichotomise on the mesial sinus and elevation, and rarely 

 on other parts of the shell. Radii about equal to the spaces 

 between them, and both are again finely striated in the 

 same direction by microscopic lines, and the whole crossed 

 by fine concentric striae which give a granulate appearance 

 to the unworn surface. 



In old shells, the proportions of length and breadth vary 

 so that they become longer than wide, the hinge line and 

 area are less than the greatest width of the shell, the valves 

 become more gibbous, the beak of the ventral valve more 

 incurved, and the area contracted : at the same time the 

 sinus becomes better defined towards the base, the mesial 

 fold distinctly elevated towards the margin, and the radii 

 more strongly developed. 



