118 MISSISSIPPIAN BRACHIOPODA 



1868. Productus magnus Meek and Worthen, Geol. Survey 111., vol. 3, p. 



528, pi. 20, figs. 7a-c. 

 1894. Productus magnus Keyes, Mo. Geol. Surv., vol. 5, p. 41. 



Description, Shell large, subhemispherical in form, broader than long, 

 the greatest width at or near the hinge-line. The dimensions of a nearly 

 complete pedicle valve are: length from hinge-line to front margin 60 

 mm., length from umbonal region to front margin 65 mm., greatest width 

 73 mm., convexity 43 mm. 



Pedicle valve gibbous, arched longitudinally with the curvature 

 usually more abrupt posteriorly, the umbonal region depressed-convex, 

 only a little protuberant beyond the hinge-line, the lateral slopes gently 

 convex, rather broadly sloping from the central portion of the valve to 

 the lateral margins, the auriculations at the cardinal extremities only 

 slightly differentiated from the general curvature of the lateral slopes; 

 mesial sinus shallow, rather narrow, rounded in the bottom, sometimes 

 almost obsolete ; beak small and strongly incurved. Surface covered with 

 somewhat obscure, depressed, rounded, radiating costae which are rather 

 slender for the size of the shell, usually about 1 mm. from center to center 

 towards the front of the valve, they continue to increase, usually by bifur- 

 cation through the entire length of the valve; more or less indistinct, 

 wrinkle-like, concentric markings occur upon the posterior slope of the 

 valve, and concentric lines of growth are present over the entire surface. 

 Rather obscure spine bases are sparsely scattered over the shell surface 

 towards the anterior and lateral margins, and a group of stronger and 

 more conspicuous ones occur near the cardinal margin, these cardinal 

 spines grow larger towards the cardinal extremities and are sometimes 

 arranged in a series of three or four oblique rows with from two to five 

 spines in each row. Interiorly the muscular scars are rather strongly 

 developed and the entire inner surface is roughened by rather fine pits, 

 the anterior and lateral margins which are in contact with the produced 

 portion of the opposite valve being smoother and rather sharply differen- 

 tiated from the visceral portion. 



Brachial valve flattened towards the beak and only gently concave 

 through the entire visceral region, curving more strongly towards the 

 anterior and lateral margins but not geniculate, the auricular portions 

 towards the cardinal extremities not differentiated from the general sur- 

 face; the mesial portion of the valve, in its anterior half, slightly ele- 

 vated in a low and narrow fold. Surface of the valve marked by radiat- 

 ing costae and concentric markings similar to those of the opposite valve. 

 Internally the large and strong cardinal process is bifid at its extremity, 

 anteriorly from its base a low, ridge-like septum extends for three-fourths 

 or more of the length of the valve, and a cardinal ridge extends laterally 

 from each side of the base of the cardinal process nearly parallel with 

 the cardinal margin; the brachial impressions are often distinctly visible 



