PALAEONTOLOGY OF IOWA. 519 



Spiriier biplicatus (n. s.). 



PLATE VII. FIG. 5 a, b. 



SHELL somewhat semicircular ; valves nearly equally con- 

 vex ; width from two and a half to three times as great as 

 the length ; hinjfe line often greatly extended, and produced 

 into mucronate points. Ventral valve convex in the centre ; 

 beak incurved ; sinus angular above, shallow and less strong- 

 ly defined below, and marked by one, two or more plications : 

 area narrow, slightly concave ; foramen broad. Dorsal valve 

 regularly convex in the middle, and somewhat flattened at 

 the extremities ; mesial fold rising but little above the pli- 

 cations on each side, from which it is separated by grooves 

 deeper than those which separate the plications themselves, 

 marked by two dichotomizing plications above, which be- 

 come four and sometimes five or six below the middle. 



SURFACE marked -by fourteen to sixteen simple rounded 

 plications on each side of the mesial lobe and sinus ; and 

 these are crossed by fine concentric undulating stride of 

 growth, which are granulose on their edges. 



I have no well preserved specimens of this species; and a single one only exhibits, 

 with some degree of perfection, the lamellose striae and granulose surface. In general 

 feature it resembles the 5 1 . marionensis ; but the specimens observed are smaller and 

 less robust, and also much more extended on the hinge line, and the shell shorter. 

 The plications are simple from their origin, except in a single specimen, where those 

 on either side of the mesial fold are dichotomized just below the beak. 



In the yellow sandstones of Burlington, and other places, where this species occurs 

 in the form of casts, it resembles the <S". mucronatus of the Hamilton group of New- 

 York; but is distinguished from that by the distinct plications of the mesial lobe 

 and sinus, and the wider area, as well as the more rounded plications and less con- 

 spicuous laminae of growth. 



Fig. 5 a. Dorsal valve from the Oolitic beds below the Burlington limestone. 

 Fig. 5 b. Cast of a dorsal valve from the Yellow sandstone. 



Geological formation and localities. In yellow sandstone and oolitic lime- 

 stone, Burlington (Iowa); in oolitic limestone, Quincy (Illinois). 



