CAMAROTCECHIA 181 



nearly a subsemicircular curve along the median line from the beak to the 

 front, and curving rather abruptly to the lateral margins; mesial fold 

 originating near the middle of the valve, only a little elevated above the 

 general surface towards the front; the beak pointed and incurved be- 

 neath that of the opposite valve ; the plications and concentric markings 

 similar in form and number to those of the opposite valve. Internally a 

 strong median septum extends anteriorly from the beak nearly or quite 

 to the center of the valve, in the rostral portion of the valve this septum 

 divides internally forming a short crural cavity between the two lateral 

 portions of the divided hinge-plate-. 



Remarks. This species has only been observed in the form of internal 

 casts from the residual Burlington chert of Knox County, Missouri, it may 

 be easily recognized by its subglobose form, its rather coarse plications 

 and its shallow mesial sinus and low fold. 



Formation. Burlington limestone (residual chert). 



CAMAROTCECHIA MUTATA (Hall) 

 Plate XXIV, Figs. 41-60 



1856. Rhynchonella mutata Hall, Trans. Albany Inst., vol. 4, p. 10. 

 1858. Rhynchonella mutata Hall, Geol. Iowa, vol. 1, pt. 2, p. 658, pi. 23, 



figs. 2a-b. 

 1882. Rhynchonella mutata Whitfield, Bull. Am. Mus. Nat. Hist., vol. 1, 



p. 52, pi. 6, figs. 43-45. 

 1883 Rhynchonella mutata Hall, 12th Rep. Geol. Surv. Ind., p. 332, pi. 



29, figs. 43-45. 



1894. Rhynchonella mutata Keyes, Mo. Geol. Surv., vol. 5, p. 103. 



1895. Pugnax mutatus Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 60, 



figs. 18-22. 



1906. Pugnax mutata Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1306, pi. 

 22, figs. 43-45. 



Description. Shell subovate or subtriangular in outline, usually wider 

 than long, the greatest width in front of the mid-length of the shell, the 

 postero-lateral margins straight or nearly straight, meeting at the beak 

 in an angle of from 80 to 100 degrees, the anterior margin truncate, the 

 antero-lateral margins more or less sharply rounded. The dimensions of 

 two individuals are : length 9.7 mm. and 8.9 mm., width 10.6 mm and 9.1 

 mm., thickness 7 mm. and 5.5 mm., width of sinus in front 7.8 mm. and 

 5.8 mm. 



Pedicle valve convex in the umbonal region, usually somewhat flattened 

 in the middle and towards the antero-lateral margins, the surface curves 

 rather abruptly to the postero-lateral margins, gently to the antero-lateral 

 margins and is arched from the beak to the front ; along the postero-lateral 

 margins towards the beak, the edge of the valve is inflected to form a 

 more or less indistinct pseudo-cardinal area, this area becoming less dis- 

 tinct in the older shells ; mesial sinus obsolete in the posterior half of the 



