364 MISSISSIPPIAN BRACHIOPODA 



originating at the beak where it is scarcely or not at all elevated above 

 the general surface of the valve, marked by plications similar to those 

 upon the lateral slopes of the valve. 



Surface of each valve marked by 80 or more depressed, rounded, 

 bifurcating plications, about 20 to 25 of which occupy the fold and 

 sinus. The minute surface markings consist of exceedingly fine radiating 

 striae, about 6 to 10 of which occupy each plication, and by still finer 

 concentric striae which give to the surface of perfectly preserved shells 

 a finely cancellated ornamentation. Concentric lines of growth are 

 usually distributed over the surface of the shell in an irregular manner, 

 often becoming somewhat crowded anteriorly. 



Remarks. This species has its nearest ally in the Mississippi basin in 

 ;!>. grimesi, these two Spirifers being the largest members of the genus 

 in America. In their surface ornamentation the two species are essen- 

 tially identical, the distinction between them being found in the greater 

 length of the hinge-line in 8. logani and in its proportionally broader shell. 

 In their internal characters the two species are also essentially alike, 

 although the muscular impressions in the pedicle valve of 8. logani are 

 sometimes proportionally broader and shorter. In their stratigraphic 

 relations 8. logani is characteristic of the Keokuk limestone, while 8. 

 grimesi is a member of the next older fauna of the Burlington limestone. 



Horizon. Keokuk limestone. 



SPIRIPER STRIATIFORMIS Meek 

 Plate XLVIII, Figs. 17-21 



1875. Spirifer (Trigonotreta) striatiformis Meek, Pal. Ohio, vol. 2, p. 289, 



pi. 14, figs. 8a-e. 

 1888. Spirifer striatiformis Herrick, Bull. Sci. Lab. Denison Univ., vol. 3, 



p. 44, pi. 1, fig. 6 ; pi. 3, figs. 23-26 ; pi. 6, figs. 6, 7 ; pi. 12, fig. 20. 

 1888. Spirifer striatiformis Herrick, Bull. Sci. Lab. Denison Univ., vol. 4, 



pi. 2, fig. 9. 

 1895. Spirifer striatiformis Herrick, Geol. Surv. Ohio, vol. 7, pi. 15, fig. 9. 



Description. Shell of medium size, wider than long, the greatest wdth 

 at or near the hinge-line, the lateral margins usually meeting the hinge- 

 line in nearly a right angle and rounding to the front, the anterior 

 margin rounding, usualry projecting a little in the middle. The dimen- 

 sions of a nearly perfect specimen are : length of pedicle valve 35 mm., 

 length of brachial valve 28 mm., width 46 mm., thickness 25 mm., height 

 of cardinal area 5 mm. 



Pedicle valve rather strongly convex, the greatest convexity posterior 

 to the middle, the umbonal region rather broad, the surface curving 

 rather abruptly to the cardinal margins for a short distance on each side 

 of the beak, curving more gently to the antero-lateral margins, somewhat 



