CHONETES 89 



by the greater extension of the hinge-line and more conspicuous auricu- 

 lations of the shell, and by the somewhat coarser costas separated by pro- 

 portionally wider intercostal furrows. 



Horizon. Glen Park limestone and Chouteau limestone of the Kinder- 

 hook. 



CHONETES SHUMARDANUS DeKoninck 

 Plate VIII, Figs. 1-7 



1847. Chonetes shumardiana DeKoninck, Monog. du Gen. Prod, et Chon., 



p. 192, pi. 20, figs. la-d. 

 1855. Chonetes Shumardiana Norwood and Pratten, Jour. Acad. Nat. Sci. 



Phil., (2), vol. 3, p. 24. 



Description. Shell semielliptical in outline, the proportional length, 

 breadth, and convexity variable, the greatest width along the hinge-line, 

 the cardinal extremities usually nearly rectangular; the lateral margins 

 nearly straight posteriorly, curving regularly into the anterior margin 

 in front. The dimensions of three nearly perfect examples are: length 

 16 mm., 12.5 mm. and 11.5 mm., width 22 mm., 15 mm. and 17 mm., con- 

 vexity 6 mm., 5.5 mm. and 4.2 mm. 



Pedicle valve varying in its convexity, but never extreme in either di- 

 rection, the greatest convexity near or a little back of the middle, com- 

 pressed and auriculate towards the cardinal extremities but with the 

 auriculations not sharply differentiated from the body of the shell ; mesial 

 sinus entirely obsolete; the beak small and inconspicuous, the umbo pro- 

 jecting a little beyond the cardinal margin; cardinal area nearly flat, 

 slightly twisted towards the cardinal extremities, lying in nearly the 

 plane of the valve ; the delthyrium about as wide as high, closed towards 

 its apex by a convex deltidium, its lower portion filled by the posterior 

 portion of the cardinal process of the opposite valve; the cardinal mar- 

 gins sharply defined, bearing the bases of from 3 to 5 oblique spines. In- 

 ternally the muscular scars are heart-shaped in outline and in the younger 

 individuals reach nearly to the middle of the valve, becoming proportion- 

 ally smaller with age, they are well developed and become rather deeply 

 impressed in old shells, a thin median septum is present which extends 

 from the beak nearly to the middle of the muscular scar, towards the car- 

 dinal extremities the valve is thickened internally, the thickened portion 

 more or less sharply differentiating a central, subovate, deeply concave 

 region from the more flattened, subtriangular regions towards the car- 

 dinal extremities; the inner surface of the valve, beyond the muscular 

 scars, covered by papillae which are scattered towards the central portion 

 of the valve, becoming much more numerous and more prominent towards 

 the lateral and anterior margins. 



Brachial valve deeply concave in the central portion, with an indistinct 

 sinus on each side extending from the center obliquely towards the car- 



