SPIRJFER 355 



SPIRIFER TENUIMARGINATUS Hall 

 Plate XL VIII, Figs. 1-5 



1858. Spirifer tenuimarginatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 641, pi. 20, 



figs. la-c. 

 1883. Spirifera tenuimarginata Hall, Rep. N. Y. State Geol. for 1882, 



pi. (32) 57, figs. 4-6. 

 1895. Spirifer tenuimarginatus Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, 



pi. 32, figs. 4, 6. 



Description. Shell small, subovate or subrhombie in outline, wider than 

 long, the greatest width at or close to the hinge-line, the hinge-line about 

 equaling the greatest width of the shell, the cardinal extremities sub- 

 angular. The dimensions of the holotype are : length of pedicle valve 19.5 

 mm., length of brachial valve 16.5 mm., width 21.8 mm., thickness 11.8 

 mm., height of cardinal area 2.5 mm. 



Pedicle valve with its greatest convexity posterior to the middle, the 

 surface a little compressed towards the cardinal extremities, the umbonal 

 region prominent, produced posteriorly somewhat beyond the hinge-line, 

 the surface sloping abruptly to the cardinal margin from the umbonal 

 region, more gently to the antero-lateral margins; beak sharply pointed, 

 narrow and incurved; cardinal area small, arched, the curvature be- 

 coming greater towards the beak, the lateral margins sharply defined, 

 sloping steeply from the beak to the cardinal extremities, the delthyrium 

 large, broader than high ; lateral slopes each marked by about 20 angular 

 plications with frequent bifurcations ; mesial sinus ill-defined anteriorly, 

 more sharply defined towards the beak, rounded in the bottom, shallow 

 or of moderate depth, marked by about 12 plications similar to those 

 on the lateral slopes, one plication originates at the median line near the 

 beak and divides into two or three before reaching the anterior margin, 

 the lateral plications originate from the inner margins of the bounding 

 plications or through the bifurcation of plications so originating. 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity near the middle, the surface compressed towards the cardinal 

 extremities ; mesial fold defined to the beak but scarcely elevated above 

 the general surface in the posterior third of the valve, bcoming strongly 

 elevated in front, rounded in contour in the middle of the shell, becoming 

 subangular towards the front, marked by plications similar in form and 

 number to those of the sinus of the opposite valve, all of which originate 

 through the division of the single original plication at the beak; lateral 

 slopes gently convex, more or less flattened towards the cardinal ex- 

 tremities, marked by angular, bifurcating plications similar to those of 

 the opposite valve. 



The minute surface markings of the shell are not well preserved, but 

 rather fine, concentric, perhaps sublamellose lines of growth are suggested, 

 with a few stronger, concentric lines of growth. 



