SPIRIFER 363 



SPIRIFER LOGANI Hall 



Plate LVI, Figs. 1-3 ; Plate LVII, Figs. 1-3 



1858. Spirifer logani Hall, Geol. Iowa, vol. 1, pt. 2, p. 647, pi. 20, fig. 7 ; 



pi. 21, figs, la-b, 2. 

 1863. Spirifer Icevigatus Swallow, Trans. St. Louis Acad. Sci., vol. 2, 



p. 86. 

 1883. Spirifera Logani Hall, Rep. N. Y. State Geol. for 1882, pi. (32) 57, 



figs. 7, 8. 

 1894. Spirifer Lo'gani Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 26, fig. 10. 



1894. Spirifera logani Keyes, Mo. Geol. Surv., vol. 5, p. 81. 



1895. Spirifer Loguni Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 32, 



figs. 7, 8. 



Description. Shell large, wider than long, the greatest width along the 

 hinge-line, the cardinal extremities usually acutely angular or acuminate, 

 sometimes rectangular. The dimensions of two individuals are: length 

 80 mm. and 61 mm., width 120 mm. and 88 mm., thickness 53 mm. and 40 

 mm., height of cardinal area 9.5 mm. and 9 mm. 



Pedicle valve moderately convex, the greatest convexity posterior to 

 the middle, the surface sloping abruptly from the broad umbonal region 

 to the cardinal margin, and rather gently to the lateral and anterior 

 margins, distinctly flattened towards the cardinal extremities; beak 

 rather obtusely pointed and incurved ; cardinal area of moderate height, 

 arched, the curvature becoming stronger towards the beak, sometimes 

 nearly or quite flat towards the hinge-line, the lateral margins sharply 

 defined, sometimes nearly parallel with the hinge-line through the greater 

 part of the length and then sloping abruptly to the cardinal extremities, 

 in other specimens sloping somewhat gradually from the beak to the 

 cardinal extremities ; the delthyrium large, broadly triangular, some- 

 times twice as wide at its base as the total height; lateral slopes convex 

 towards the median part of the valve, becoming concave towards the 

 cardinal extremities, covered with depressed, rounded, bifurcating plica- 

 tions from 1 to 2 mm. in width; mesial sinus broad and usually rather 

 shallow, rounded in the bottom, originating near the beak, its lateral 

 boundaries indefinitely marked, produced into a nasute extension anter- 

 iorly, its surface covered with bifurcating plications similar to those 

 upon the sides of the shell. 



Brachial valve about as convex as the pedicle, its greatest convexity 

 near the middle ; the lateral slopes convex towards the middle part of 

 the shell, becoming concave or flattened towards the cardinal extremi- 

 ties, marked by depressed, rounded, bifurcating plications like those on 

 the opposite valve; mesial fold broad, rounded, ill-defined, sometimes 

 becoming strongly elevated towards the front and somewhat angular, 



