262 MISSISSIPPIAN BRACHIOPODA 



Remarks. This species is a rare one and has not been available in suf- 

 ficient numbers to make it possible to grind any examples for the purpose 

 of determining its internal characters. The interior of the pedicle valve, 

 however, is perfectly shown in one of the cotypes, and the essential fea- 

 tui es of the genus Dielasma can be recognized from the external surface of 

 the brachial valve of another cotype, the position of the crural lamellae 

 and the transverse muscle plate being clearly indicated by dark lines. 

 In size, and in the entire absence of fold and sinus, the species resembles 

 D. formosa, but it is a somewhat more slender shell with less convex valves 

 and with the beak of the pedicle valve less incurved; the species is es- 

 pecially characterized, however, by the large size of the foramen which is 

 directed straight posteriorly or even obliquely away from the brachial 

 valve, while in D. formosa the foramen is directed obliquely postero- 

 brachially. The concentric growth lines of the shell are also much less 

 conspicuous in this species than in D. formosa. 



Horizon. Burlington limestone. 



DIELASMA SINUATA n. sp. 

 Plate XXXI, Figs. 28-34 



1883. Lingula crawfordsvillensis Gurley, New Carb. Foss., Bull. No. 1, 



P. a. 



1895. Dielasma formosa Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 81, 



figs. 16-17 (not figs. 12-15, 18-26). 

 1906. Dielasma turgidum Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1309, 



pi. 19, figs. 5-5a (not pi 22, figs. 53-58). 

 1906. Dielasma gorbyi Beede, 30th Ann. Rep. Geol. Surv. Ind., p. 1310, 



pi. 20, fig. 5 (not Terebratula gorbyi Miller). 



Description. Shell of medium size, subovate or ovate-subpentagonal in 

 outline, longer than wide, the greatest width near the mid-length of the 

 shell, the anterior margin rounded, nearly straight or a little emarginate. 

 The dimensions of an internal cast, somewhat restored, are: length of 

 pedicle valve 29.2 mm., length of brachial valve 23.6 mm., greatest width 

 18.7 mm., thickness 20.9 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the umbonal region projecting conspicuously beyond the beak of 

 the opposite valve in full grown specimens, the surface arched from beak 

 to front with the curvature becoming progressively more convex poste- 

 riorly, the lateral slopes curving abruptly and steeply from the median 

 portion of the valve to the lateral margins, becoming somewhat inflected 

 towards the cardinal extremities, the lateral margins joining those of the 

 opposite valve in a broadly obtuse angle ; mesial sinus originating near 

 the middle of the valve, rather narrow, with subparallel sides, rounded 

 or a little flattened in the bottom, its lateral borders rounding into the 

 lateral slopes of the valve, of moderate depth or becoming strongly de- 



