70 MISSISSIPPIAN BBACHIOPODA 



occupy the space of one millimeter, these costse increase by both bifurca- 

 tion and intercalation and are crossed by concentric markings similar to 

 those of the opposite valve but considerably fainter. 



Remarks. This species most resembles the specimens of 8. ruginosa from 

 the Salem and St. Louis limestone, both forms being characterized by the 

 short hinge-line and by the strong, irregular, concentric wrinkles of the 

 pedicle valve, but the brachial valve is much more regular in contour in 

 this species than in the S. ruginosa. The species under discussion may be 

 easily distinguished from 8.- ruginosa by the slender costee of the pedicle 

 valve with the broad intercostal furrows and by the character of the fine 

 concentric markings, also by the difference in character of the radiating 

 costae of the two valves. 



Horizon. Chouteau limestone. 



STREPTORHYNCHUS MINUTUM (Cumings) 

 Plate VI, Figs. 16-21 



1856. Orthis umbraculum Hall, Trans. Albany Inst., vol. 4, p. 12. 



1883. Streptorhynchus (Orthis) Umbraculum Hall, 12th Rep. Geol. Surv. 



Ind., p. 325. 

 1901. Orthothetes minutus Cumings, Am. Geol., vol. 27, p. 147, pi. 15, 



figs. 1-16. 



1906. Orthothetes minutus Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1297, 

 pi. 18, figs. 1-16. (Not pi. 20, fig. 7). 



Description. Shell very small, more or less distorted, wider than long, 

 subelliptical to subquadrate in outline, the hinge-line a little shorter 

 than the greatest width, the cardinal extremities angular. The dimensions 

 of a small, complete specimen are : length of pedicle valve 2.1 mm., length 

 of brachial valve 2 mm., greatest width 3.1 mm., length of hinge-line 2.9 

 mm., height of cardinal area .9 mm. The dimensions of a larger pedicle 

 valve are : length 5.8 mm., greatest width 8.5 mm., length of hinge-line 

 7 mm., height of cardinal area 1.8 mm. 



Pedicle valve usually convex in young examples, becoming concave 

 towards the front margin with the continued growth of the shell, the 

 surface most prominent at the apex of the valve, from which point it 

 slopes in nearly straight, gently concave or gently convex lines anteriorly 

 and laterally, always becoming concave towards the front in adult speci- 

 mens; mesial sinus obsolete; beak obtusely pointed, usually nearly erect, 

 sometimes a little recurved or slightly incurved ; cardinal area of moderate 

 height, usually nearly flat but sometimes a little concave or convex, the 

 lateral margins usually sharply defined, sometimes rounding into the 

 lateral slopes of the valve ; the delthyrium triangular, about as wide as 

 high, covered by a convex deltidium. Internally the cardinal teeth are 

 widely divergent, the dental plates are restricted to slight ridge-like 

 thickenings of the inner surface of the cardinal area on each side of the 

 delthyrium, the muscular scars are ill-defined and inconspicuous. 



