PRODUCTUS 127 



of the cardinal process a low median ridge or septum reaches beyond 

 the middle of the valve. 



Remarks. This species has its nearest ally in P. marginidnctus but it 

 differs markedly from that species in the much less development of 

 the inflated, marginal border of the pedicle valve, in the finer radiating 

 costae and the less conspicuous concentric, wrinkle-like markings of the 

 posterior portion of the shell, and in the absence of a median sinus of 

 the pedicle valve. The brachial valves are more nearly alike, both being 

 nearly flat throughout most of their width and length, with the abruptly 

 deflected lateral and anterior margins, but the surface markings of the 

 valve in P. wortheni are somewhat finer than in P. marginidnctus. 



Horizon. Keokuk limestone. 



PRODUCTUS MARGINICINCTUS Prout 

 Plate XIII, Figs. 18-23 



1857. Productus marginidnctus Prout, Trans. St. Louis Acad. Sci., vol. 1. 



p. 43, pi. 2, figs. 1-16. 



1858. Productus marginidnctus Hall, Geol. Iowa, vol. 1. pt. 2, p. 674, 



pi. 24, figs. 3a-d. 

 1894. Productus marginidnctus Keyes, Mo. Geol. Surv., vol. 5, p. 43. 



Description. Shell below medium size, usually broader than long, the 

 hinge-line shorter than the greatest width, the greatest width anterior 

 to the mid-length of the shell. The dimensions of a specimen of average 

 size are: length from hinge-line to anterior margin 19.3 mm., length 

 from umbonal region to anterior margin 22 mm., width 24.5 mm., con- 

 vexity of pedicle valve 15 mm., length of hinge-line 18 mm. 



Pedicle valve gibbous, the umbo prominent and protuberant beyond 

 the hinge-line, the surface curving abruptly towards the margins, com- 

 pressed towards the cardinal extremities in small auriculations, laterally 

 and anteriorly the surface is abruptly deflected near the margin and then 

 convexly curved and again inflected to form a conspicuous, ring-like or en- 

 rolled border; mesial sinus shallow and ill-defined, sometimes nearly or 

 quite obsolete; beak rather small and pointed, incurved. Surface of 

 the valve marked by rather strong, rounded costae, which increase 

 by bifurcation and intercalation, and which continue with only slight 

 interruption across the ring-like marginal border; on the anterior por- 

 tion of adult specimens just back of the marginal border the costas 

 are usually about one millimeter apart from center to center, the dis- 

 tances varying, however, from .5 to 1.5 mm., posteriorly and towards 

 the cardinal extremities the costae are finer; at more or less irregular 

 intervals the costae are distinctly nodose, and from each node a slender, 

 elongate spine is produced anteriorly, tangential to the surface, the 

 spines becoming more numerous upon the marginal border ; ' crossing 



