CHONETES 81 



CHONETES ILLINOISENSIS Worthen 

 Plate VIII, Figs. 63-70 



1858. Chonetes logani Hall, Geol. Iowa, vol. 1, pt. 2, p. 598, pi. 12, figs. 



la-e, 2. (Not C. logani N. & P., 1855.) 

 1860. Chonetes Illinoiensis Worthen, Trans. St. Louis Acad. Sci., vol. 1, 



p. 571. 



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

 1865. Chonetes Illinoisensis Winchell, Proc. Acad. Nat. Sci. Phil., p. 116. 

 1868. Chonetes Illinoisensis Meek and Worthen, Geol. Surv. 111., vol. 3, 



p. 505, pi. 15, figs. 8a-b. 

 J870. Chonetes Illinoisensis Winchell, Proc. Amer. Phil. Soc., vol. 11, 



p. 251. 

 1888. Chonetes illinoisensis Herrick, Bull. Sci. Lab. Denison Univ., vol. 3, 



p. 35, pi. 3, fig. 21. 

 1890. Chonetes illinoisensis Herrick, Bull. Geol. Soc. Amer., vol. 2, p. 48, 



pi. 1, fig. 16. 



1894. Chonetes illinoisensis Keyes, Mo. Geol. Survey, vol. 5, p. 53. 

 1903. Chonetes illinoisensis Girty, U. S. Geol. Survey, Prof. Paper No. 16, 



p. 279. 



Description. Shell concavo-convex, the length about two-thirds the 

 width, the hinge-line usually a little shorter than the maximum width, 

 the hinge extremities a little rounded or nearly rectangular, lateral mar- 

 gins slightly convex or nearly straight, more strongly curved anteriorly 

 and passing with a regular curvature into the anterior margin, which 

 becomes nearly straight for a short distance in the middle portion. The 

 dimensions of a specimen of about average size are: length 11.9 mm., 

 width 16.3 mm., convexity 3.6 mm. 



Pedicle valve depressed convex, the greatest convexity posterior to the 

 middle, compressed towards the cardinal extremities, flattened along the 

 mesial region and sometimes depressed in a shallow, ill-defined mesial 

 sinus ; the beak small, scarcely extended beyond the cardinal margin ; 

 cardinal area narrow, slightly concave, lying at an angle of about 145 

 degrees to the plane of the valve, its lateral margins sharply defined and 

 bearing five or six oblique spines upon each side ; the delthyrium broadly 

 triangular, about twice as wide as high, the deltidium small, confined 

 to the apex and margins of the delthyrium. 



Brachial valve only slightly concave, a little flattened towards the car- 

 dinal margins ; the cardinal area not well shown on any of the authentic 

 specimens, but it is probably small. Internally the cardinal process is of 

 moderate size, and from its anterior side a pair of ridges of variable 

 strength diverge widely, becoming nearly parallel with the cardinal mar- 

 gin and extending nearly half way to the cardinal extremities ; a second 

 pair of ridges originate in front of the cardinal process, border the basal 

 portion of the muscular sear, diverging with an angle of about 50 degrees, 



