SFIRIFER 323 



by slightly coarser radiating striae which are more conspicuous in the 

 furrows between the plications than upon the plications themselves. 



Remarks. This species may be easily recognized by its deep pedicle 

 valve, its narrow umbonal region, and narrow fold and sinus. The minute 

 surface markings have been observed only upon specimens from the white 

 cherts at Louisiana, Missouri. 



Horizon. Upper Kinderhook and lower Burlington limestone. 



SPIRIFER BIPLICOIDES n. sp. 



Plate XXXIX, Figs. 27-30 

 1858. Spirifer biplicatus Hall, Geol. Surv. Iowa, vol. 1, pt. 2, p. 519, pi. 7, 



fig. 5a (not fig. 5b). 

 1901. Spirifer centronatus Weller, Trans. St. Louis Acad. Sci., vol. 11, 



p. 163, pi. 14, figs. 34. 



Description. Shell small, much broader than long, the greatest width 

 along the hinge-line, the cardinal extremities acuminate. The dimensions 

 of a pedicle valve are: length 11 mm., width along the hinge-line 32 mm., 

 convexity 5.5 mm. The dimensions of a brachial valve are : length 8 mm., 

 width along the hinge-line 25 mm., convexity 3.5 mm. 



Pedicle valve strongly convex from beak to front, the greatest convex- 

 ity near or posterior to the middle of the shell, the surface compressed 

 towards the cardinal extremities which are produced into more or less 

 elongate, attenuate points ; beak small, pointed, incurved ; cardinal area 

 narrow, arched, the lateral margins sharply defined, sloping from the 

 beak to the cardinal extremities ; lateral slopes convex antero-posteriorly, 

 becoming compressed distally, each marked by from 12 to 16 simple, 

 rounded plications; mesial sinus narrow and shallow, defined to the beak, 

 the bounding plications much broader than any of the plications of the 

 lateral slopes, marked by a low and narrow median plication which orig- 

 inates in the umbonal region and continues without division to the ante- 

 rior margin, a little in front of the point of origin of the median plication 

 each of the broad, bounding plications is divided by a narrow groove much 

 weaker than those between the plications of the lateral slopes, the inner 

 division of the plication occupying the lateral slopes of the sinus. 



Brachial valve less convex than the pedicle, the greatest convexity near 

 or posterior to the middle, compressed towards the cardinal extremities; 

 mesial sinus sharply defined by a pair of furrows which are distinctly 

 broader and stronger than those between the other plications, it is de- 

 pressed and not elevated above the general surface of the valve posteriorly 

 and is but slightly elevated anteriorly, it is simple at the beak, is divided 

 by a median furrow in the umbonal region and further towards the front 

 in mature shells each of the lateral divisions is again subdivided, all the 

 furrows of the fold being weaker than those which separate the plications 



