50 MISSISSIPPIAN BRACHIOPODA 



3895. Strophomena, rhomboidalis Herrick, Geol. Surv. Ohio, vol. 7, pi. 20, 



fig. 6. 



1899. Leptcena rhomboidwlis Girty, Mon. U. S. Geol. Survey, vol. 32, p. 525. 

 1904. Leptmna rhomboidalis Girty, Prof. Paper U. S. Geol. Survey, No. 21, 



p. 48, pi. 10, fig. 3. 

 1909. Leptaena rhomboidalis Waller, Bull. Geol. Soc. Am., vol. 20, p. 292, 



pi. 12, figs. 2-3. 



Description. Shell of medium size or larger, subtrapezoidal in outline, 

 wider than long, the greatest width along the hinge-line, the cardinal ex- 

 tremities angular, sometimes rather broadly auricular. The dimensions 

 of a nearly complete example are : length 25 mm., width along hinge-line 

 estimated 40 mm. 



Pedicle valve depressed convex in the umbonal region, flattened ante- 

 riorly and laterally to near the margin of the valve where it is abruptly 

 genieulate, the deflected portion variable in width with age, sometimes 

 having a width equal to more than one-half the length of the flattened 

 portion of the valve. The surface of the valve at the genieulate margin is 

 sometimes raised in an elevated border which is highest in the median 

 portion of the valve in front, becoming gradually reduced along the 

 lateral margins anteriorly and becoming obsolete towards the cardinal 

 extremities ; the deflected border of the valve is sometimes sinuate in front, 

 the sinus when present is broad, ill-defined, and shallow, and does not 

 continue across the flattened portion of the valve ; beak small, slightly or 

 not at all extended beyond the cardinal margin, and not incurved, usually 

 pierced by a minute, subcircular foramen which often becomes filled in 

 mature shells ; cardinal area narrow, flat or slightly concave, sloping pos- 

 teriorly from the hinge-line and lying at an angle of 45 degrees or less to 

 the plane of the valve, the lateral margins sharply defined ; delthyrium 

 broadly triangular, closed at its apex by a small deltidium, the greater 

 portion of it filled by the cardinal process of the opposite valve. Intern- 

 ally the cardinal teeth are rather thick and short ; near or a little poste- 

 rior to the middle of the lateral margins, a rather narrowly elevated ridge 

 passes posteriorly with a gentle curvature towards the middle of the 

 hinge-line, the narrowly triangular areas toward the cardinal extremities, 

 which are isolated by these ridges, are rather deeply excavated. The mus- 

 cular scar is deeply concave, broader than long, subrhomboidal to sub- 

 elliptical in outline, and is surrounded laterally and anteriorly by an 

 angular ridge which terminates on either side just in front of the cardinal 

 teeth ; the length of the scar is one-third or less than one-third the total 

 length of the valve, it is divided along its median line by a low angular 

 ridge originating just within the anterior border and becoming obsolete 

 towards the posterior border, on each side of this ridge are the narrow 

 and elongate adductor scars, beyond which the much larger diductor scars 

 occupy the greater portion of the total area; the primary trunk of the 



