120 MISSISSIPPIAN BRACHIOPODA 



half millimeter apart, which increase somewhat regularly by bifurcation 

 and intercalation upon the posterior slope, often becoming more or less 

 wavy or flexuose upon the anterior and antero-lateral slopes and increasing 

 irregularly by intercalation or bifurcation, or sometimes anastomosing; 

 upon the posterior slope of the valve there are rather regular, concentric, 

 wrinkle-like markings which vary in strength upon different individuals 

 and usually become stronger towards the cardinal margins; fine concen- 

 tric lines of growth cover the entire surface when it is perfectly pre- 

 served ; spine bases rare or wholly absent upon the body of the valve, but 

 indications of a small group of spines of moderate size i~re frequently 

 present upon the auriculations near the cardinal margin. 



Brachial valve not observed. 



Remarks. This species is characterized by the fine radiating costas 

 and by the thinness of the shell substance. To this last character may 

 be due the irregular and more or less flexuous growth of the costae upon 

 the anterior and lateral slopes of the valve because of its special liability 

 to injury during growth. The flaring of the pedicle valve towards its 

 outer margin, which is sometimes present, is perhaps due to deformation 

 of the shell during fossilization, again because of its delicacy. In some 

 respects the members of this species resemble P. ovatus, but its relation- 

 ship is rather with the semireticulatus type of the genus than with the 

 cora type, because of the much more continuous, concentric, wrinkle-like 

 markings of the posterior slope of the shell. The shell resembles P. ovatus 

 in its delicacy and in its fine, more or less flexuous costae, but in that 

 species and its allies the cross wrinkles are confined closely to that portion 

 of the valve adjacent to the cardinal extremities. 



Horizon. St. Louis limestone. 



PRODUCTUS SCITULUS Meek and Worthen 

 Plate XIII, Figs. 24-29 



1860. Productus scitulus Meek and "Worthen, Proc. Acad. Nat. Sci. Phil,, 



p. 451. 

 1866 Productus scitulus Meek and Worthen, Geol. Surv. 111., vol. 2. 



p. 280. pi. 20, figs. 5a-d. 



Description. Shell small, usually broader than long but becoming a 

 little longer than wide in old individuals, the hinge-line about equaling 

 the greatest width of the shell, the anterior margin regularly rounded. 

 The dimensions of a large and old example are: length from hinge-line 

 to front margin 9.3 mm., length from umbonal region to front margin 

 13.5 mm., greatest width 13 mm., convexity of pedicle valve 8.1 mm. 



Pedicle valve gibbous, arched from the umbonal region to the front 

 with the posterior curvature shorter and a little more abrupt than the 

 anterior, the umbonal region strongly protuberant beyond the hinge-line, 



