308 MISSISSIPPIAN BRACHIOPODA 



vertically striated ; the muscular impressions much less strongly marked 

 than in the pedicle valve; the dental sockets narrow and of moderate 

 depth, the socket plates well developed and at their extremities sup- 

 porting the crural bases ; the crura are long, straight, and slightly diverg- 

 ent, the spiral cones are directed obliquely outward and posteriorly 

 towards the cardinal extremities, the primary lamellae are not united by a 

 jugum, but the position of the jugum is indicated by the presence of a 

 pair of spine-like processes upon the primary lamellae a little in front of 

 their junction with the erura. 



Remarks. This genus has the largest specific representation of any in 

 our Mississippian faunas, and the species themselves are usually very 

 characteristic of the horizon in which they occur. As a whole the Missis- 

 sippian species are characterized by the presence of plications in the fold 

 and sinus of the valves, and in a large number of the species the plica- 

 tions upon the lateral slopes exhibit more or less bifurcation. 



SPIRIFEB MARIONENSIS Shumard 

 Plate XXXVII, Figs. 1-7 



1855. Spirifer Marionensis Shumard, Geol. Rep. Mo., 1855, p. 203, pi. C, 



figs. 8a-b (not fig. 8c). 

 1858. Spirifer marionensis Hall, Geol. Surv. Iowa, vol. 1, pt. 2, p. 511, 



pi. 6, figs. la-c. 

 1883. Spirifera Marionensis Hall. 2nd Ann. Rep. N. Y. State Geol., pi. 



(31) 56, fig. 15. 

 1894. Spirifer Marionensis Hall and Clarke, 13th Ann. Rep. N. Y. State 



Geol., vol. 2, pi. 27, fig. 3. 

 1894. Spirifera- maricnensis Keyes, Mo. Geol. Surv., vol. 5, p. 78, pi. 40, 



figs. la-b. 

 .1895. Spirifer Marionensis Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 31, 



fig. 15. 

 1908. Spirifer marionensis Rowley, Mo. 'Bureau Geol. and Mines, vol. 8, 



2nd ser., p. 81, pi. 18, figs. 1-5; p. 88, pi. 19, figs. 3-6. 



Description. Shell of medium size, subsemicircular to subtriangular in 

 outline, greatest width usually along the hinge-line, cardinal extremities 

 acutely angular in immature shells, the hinge-line becoming relatively 

 shorter and the cardinal extremities more obtuse with age. The dimen- 

 sions of an average specimen are : length of pedicle valve 24 mm., length 

 of brachial valve 20 mm., width along hinge-line 39 mm., thickness 17 mm., 

 height of cardinal area 3 mm. 



Pedicle valve moderately convex, usually becoming compressed towards 

 the cardinal extremities, the umbonal region rather narrow, the surface 

 curving abruptly from it to the cardinal margin for a short distance on 

 each side of the beak, the slope more gentle to the antero-lateral margins ; 

 beak small, pointed, and incurved ; cardinal area of moderate height, 

 arched, the curvature increasing towards the beak, the lateral margins 



