368 MISSISSIPPIAN BRACHIOPODA 



distally, the slope becoming much more abrupt as it approaches the car- 

 dinal extremities; delthyrium large, broader than high; lateral slopes 

 with a more or less distinct sinuosity extending obliquely from each side 

 of the beak to the lateral margins just in front of the cardinal extremi- 

 ties, each marked with about 25 rounded or subangular plications, 5 to 8 

 of which towards the sinus are double, with about 10 additional single 

 ones towards the cardinal extremities, the last 3 or 4 of which are but 

 faintly developed, the double plications bifurcate in the umbonal region 

 and continue in pairs to the anterior margin, the grooves between the 

 pairs being distinctly wider and deeper than those between the two pli- 

 cations of each pair ; mesial sinus distinctly defined from the beak to the 

 anterior margin, subangular in the bottom, marked by a median plication 

 which originates near the beak and continues without division to the an- 

 terior margin, on each side of the sinus there are 3 or 4 plications which 

 originate from the inner margin of the bounding plications or from the 

 bifurcation of such plications. 



Brachial valve not observed. 



The surface markings of the shell consist of fine, sublamellose, concen- 

 tric lines which are crowded at intervals to form more or less conspicuous 

 lines of growth. 



Remarks. This species is more closely allied to S. mortonensis than to 

 any other, but it may be easily separated from that species by means of 

 the distinctly duplicate plications which occupy a considerable area on 

 each side of the sinus, and by the more sharply defined mesial sinus of 

 the pedicle valve. 



Horizon. Keokuk (Harrodsburg limestone of Indiana). 



SPIRIFER CALVINI n. sp. 

 Plate LIV, Figs. 5-10 



Description. Shell of medium size, wider than long, the greatest width 

 at or near the hinge-line, the cardinal extremities angular or a little 

 rounded. The dimensions of a nearly complete individual, slightly dis- 

 torted in such a manner as to reduce the length and increase the thickness, 

 are : length of pedicle valve 29 mm., length of brachial valve 24.5 mm., 

 width 45.5 mm., thickness 30.5 mm., height of cardinal area 8 mm. 



Pedicle valve subpyradimal in form, the umbo prominent ; beak pointed, 

 more or less incurved, sometimes a little twisted ; cardinal area high, con- 

 cave, marked by longitudinal lines of growth, the lateral margins defined 

 but sometimes curving rather abruptly into the lateral slopes, sloping 

 from the beak to the cardinal extremities in a slightly convex curve, the 

 delthyrium large, broadly triangular; lateral slopes convex, sloping 

 steeply from the umbonal region to the antero-lateral margins, each 

 marked by from 12 to 14 simple, depressed convex, plications which are 



