PRODUCTUS 121 



the median portion of the valve gently convex transversely, curving more 

 strongly into the lateral slopes which drop more or less abruptly to the 

 lateral margins, auriculations at the cardinal extremities rather sharply 

 differentiated, of moderate size, rather strongly convex antero-posteriorly : 

 the median sinus usually obsolete, but in occasional examples a slight 

 median depression occurs; the beak strongly incurved. Surface of the 

 valve marked by fine, radiating costae, about three of which occupy the 

 space of 1 mm., they increase by bifurcation and intercalation on the 

 posterior slope of the valve but continue with few or no divisions across 

 the anterior slope, on occasional examples several groups of two or three 

 costge are raised slightly above the general surface upon the anterior 

 slope ; crossing the radiating costse upon the posterior slope are more or 

 less well-defined, concentric, wrinkle-like markings which are commonly 

 stronger toward the cardinal margin: when the surface is not exfoliated, 

 minute, concentric lines of growth are present. Spine bases are commonly 

 inconspicuous, but a few rather widely scattered ones are sometimes 

 present, usually upon the anterior slope. 



Brachial valve not observed. 



Remarks. This species is a commion one in the St. Louis limestone 

 and may be easily recognized by its small size and its fine, radiating 

 costge. 



Horizon. St. Louis limestone. 



PRODUCTUS PARVUS Meek and Worthen 

 Plate XVI, Figs. 16-22 



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



p. 450. 

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



p. 297, pi. 23, figs. 4a-e. 

 1888. Productus parvus Herrick, Bull. Sci. Lab. Denison Univ., vol. 4, pi. 



11, fig. 11. 



Description. Shell small, length and width usually nearly equal, the 

 greatest width near the mid-length of the shell, the hinge-line about 

 equaling the greatest width. The dimensions of a nearly perfect specimen 

 are: length from umbonal region to front margin 14 mm., length from 

 hinge-line to front margin 10.5 mm., greatest width 14.8 mm., length of 

 hinge-line 14 mm., convexity of pedicle valve 8.5 mm., depth of visceral 

 cavity between the two valves 5.2 mm. 



Pedicle valve gibbous, the umbonal region broad and conspicuously 

 protuberant beyond the hinge-line, mesial portion of the valve from the 

 umbonal region to the front broadly flattened or gently convex trans- 

 versely, the surface of the valve arched from the umbonal region to the 

 front margin with the top of the arch usually situated posterior to the 

 middle, the lateral slopes curving abruptly to the margins, the auricula- 



