CHONETES 79 



internal surface papillose as in the opposite valve. External surface of 

 both valves marked by usually fine, radiating costae, sometimes concen- 

 trically marked also, rarely nearly smooth. 



Remarks. This easily recognized genus is represented by numerous 

 species and an abundance of individuals in the lower Mississippian faunas, 

 but after the close of the Osage epoch it becomes exceedingly rare. The 

 spines of the cardinal margin, which are so characteristic of the genus 

 are rarely well preserved, but their bases commonly can be detected 

 upon most fairly preserved specimens. 



CHONETES MULTICOSTA Winchell 

 Plate VIII, Figs. 8-16 



1863. Chonetes multicosta Winchell, Proc. Acad. Nat. Sci. Phil., p. 5. 



1869. Chonetes multicosta Winchell, Safford's Geol. Tenn., p. 443. 



1870. Chonetes multicosta Winchell, Proc. Am. Phil. Soc., vol. 11, p. 250. 

 1909. Chonetes illinoisensis Weller, Bull. Geol. Soc. Am., vol. 20, p. 297, 



pi. 12, fig. 11. 



Description. Shell concavo-convex, length two-thirds, or a little less, of 

 the width, the greatest width usually at the hinge-line but in some speci- 

 mens the hinge-line is a little shorter than the greatest width, the cardinal 

 extremities usually nearly rectangular, the lateral margins straight or 

 slightly convex posteriorly, becoming regularly convex anteriorly and 

 passing with unbroken curvature into the anterior margin which becomes 

 gently convex or nearly straight in its median portion. The dimensions 

 of Winchell's type specimen, a pedicle valve, are: length 13 mm., width 

 20.5 mm., convexity 3.5 mm. 



Pedicle valve depressed-convex, the greatest convexity posterior to the 

 middle, compressed towards the cardinal extremities, the surface flattened 

 along the mesial region and usually slightly depressed in a rather broad, 

 shallow, ill-defined mesial sinus in the anterior two-thirds of the shell ; 

 beak small, very slightly produced beyond the cardinal margin ; cardinal 

 area narrow, slightly concave, lying at an angle of about 140 degrees to 

 the plane of the valve, its lateral margins sharply defined and bearing 

 about five oblique spines on each side of the beak ; the delthyrium broadly 

 triangular, much wider than high. The inner surface of the valve finely 

 papillose beyond the middle, the papillee appearing as small pits upon 

 the surface of internal casts of the valve. 



Brachial valve only moderately concave, somewhat flattened towards 

 the cardinal extremities. The cardinal area not well shown on any of 

 the examples examined, but it is probably small. The internal characters 

 not observed. 



Surface of both valves marked throughout by fine, usually regular, 

 radiating costaj, which increase by bifurcation on the pedicle valve and 



