PRODUCTUS 119 



and lie anterior to the middle of the valve ; subparallel with the anterior 

 and lateral margins a rugose ridge formed by the thickening of the inner 

 surface of the valve differentiates the visceral portions from the portion 

 which is produced in contact with the opposite valve. 



Remarks. This is one of the largest of our species of Productus and is 

 readily distinguished from any other here described by its subhemispheri- 

 cal form, its slightly protuberant umbo of the pedicle valve, its broadly 

 sloping lateral slopes and its slightly differentiated auriculations. The 

 arrangement of spine bases in oblique rows along the cardinal margin, as 

 described, has been clearly observed in several examples, in all the speci- 

 mens studied in which the preservation is sufficiently good to exhibit the 

 character of these spines, and it is not unlikely that this arrangement is 

 characteristic of the species. The shell described by Swallow as 

 P. fentonensis is clearly identical with P. magnus, although no examples 

 from Fenton, Missouri, have been seen which are as large as the larger 

 examples of P. ma-gnus from Monroe County, Illinois, the type locality for 

 the species. 



Horizon. Upper Keokuk limestone. 



PRODUCTUS TENUICOSTUS Hall 

 Plate X, Figs. 25-27 ; Plate XI, Figs. 1-5 



1858. Productus tenuicostus Hall, Geol. Iowa, vol. 1, pt. 2, p. 675, pi. 24, 



figs. 2a-d. 

 1883. Productus tenuicostus Hall, Rep. N. Y. Stale Geol. for 1882, pi. 



(18) 49, fig. 18. 

 1892. Productus tenuico status Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 



pi. 18, fig. 18. 

 1894. Productus tenuicostus Keyes, Mo. Geol. Surv., vol. 5, p. 44. 



Description. Shell thin, of medium size, length and width subequal or 

 longer than wide, the hinge-line shorter than the greatest width. The 

 dimensions of an incomplete pedicle valve are: length from hinge-line 

 to front margin 25 mm., length from umbonal region to front margin 

 31 mm., greatest width 28 mm., length of hinge-line 24 mm., con- 

 vexity 14 mm. 



Pedicle valve gibbous, the greatest convexity posterior to the middle, 

 the umbonal region broad and protuberant beyond the hinge-line, the 

 surface gently convex transversely or slightly sinuate in the median re- 

 gion for about one-half the total width of the valve, the lateral slopes 

 curving abruptly to the lateral margins and usually a little inflected to 

 the cardinal margin, the anterior slope with a much more gentle curva- 

 ture, the valve sometimes flaring slightly towards the lateral and anterior 

 margins, towards the cardinal extremities the valve is compressed some- 

 what abruptly to form rather prominent auriculations. Surface marked by 

 fine, more or less irregular, rounded, radiating costse usually about one- 



