SPIRIFER 335 



characterized by its notably coarser and more angular plications. In the 

 form of its plications and the finer concentric markings, 8. imbrex resem- 

 bles S. vernonensis,ihe larger and more transverse examples of S.vernonensis 

 especially being suggestive of this species, but 8. vernonensis has a more 

 elevated mesial fold anteriorly than 8. imbrex, and the mesial sinus is 

 probably much deeper, the plications also are somewhat coarser. 

 Horizon. Burlington limestone. 



SPIRIFER INCERTUS Hall 



Plate XLI, Figs. 6-14 



1858. Spirifer incertus Hall, Geol. Iowa, vol. 1, pt. 2, p. 602, pi. 13, fig. 3. 



Description. Shell of medium size, about twice as wide as long, the 

 greatest width along the hinge-line, the cardinal extremities angular. The 

 dimensions of two pedicle valves are : length 36 mm. and 28.5 mm., 

 width 65 mm. and 55 mm., convexity 12 mm and 11 mm., height of cardinal 

 area 5 mm. and 4 mm. The dimensions of a nearly complete specimen 

 are : length of pedicle valve 31.5 mm., length of brachial valve 29 mm., 

 width along hinge-line 56 mm., thickness 22.5 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the umbonal region broad, the surface sloping rather gently 

 to the margins except to the cardinal margin for a short distance on each 

 side of the beak; beak small, pointed, strongly incurved; cardinal area 

 rather narrow, strongly arched, the superior portion from the beak well 

 out towards the cardinal extremities being more strongly and abruptly 

 curved, the inferior portion in adult examples directed at an angle of about 

 90 degrees to the plane of the valve, the lateral margins sharply defined, 

 subparallel with the cardinal margins through a greater part of the length, 

 sloping abruptly to the cardinal extremities distally, delthyrium large, 

 broadly triangular ; lateral slopes of the valve convex towards the median 

 part of the valve, somewhat compressed towards the cardinal extremities, 

 each bearing about 25 subangular or rounded plications at the margin, 

 which originate through bifurcation from a much smaller number at the 

 beak, the bifurcations mostly taking place in the umbonal region, only 

 occasionally in the middle or anterior portion of the shell; mesial sinus 

 narrow, simple and sharply defined at the beak, becoming ill-defined an- 

 teriorly where it is rather broad and of moderate depth, rounded or sub- 

 angular in the bottom, marked by a median plication which originates 

 near the beak and continues to the front margin without division, with 4 

 or sometimes 5 plications on each side which originate from the inner mar- 

 gin of the lateral bounding plication or through the bifurcation of plica- 

 tions originating in that manner, all the plications of the sinus are similar 

 in form and size to those of the lateral slopes. 



