ooG MISSISSIPPIAN BRACHIOPODA 



Remarks. Tim species is known to the writer only through a sulphur 

 east of the holotype. The entire aspect of this specimen strongly suggests 

 a small and exceptionally narrow example of 8. tenuicostatus, and it is 

 not, unlikely that such an interpretation of the specimen may be correct. 

 Examples of 8. rcsteUatus have sometimes been identified as S. tenuimar- 

 ginatus, due to the fact that the figured type of S. rostellatus is an un- 

 usually narrow specimen of the species. The type of S. tenuimarginatus 

 differs notably from S. rostellatus in its much finer and more angular plica- 

 tions and its proportionally much more elongate hinge-line. 



Horizon. Keokuk limestone. 



SPIRIFER SUBROTUNDUS n. nom. 

 Plate L, Figs. 15-22 



1858. Spirifer subrotundatus Hall, Geol. Surv. Iowa, vol. 1, pt. 2, p. 521, 

 pi. 7, fig. 8. (Not S. subroiundatus McCoy, 1855.) 



1900. Spirifer subrotundatus Weller, Trans. St. Louis Acad. Sci., vol. 10, 

 p. 76, pi. 2, figs. 8-10. 



Description. Shell of medium size, subcircular, subovate or subellipti- 

 cal in outline, the width equal to, greater than, or less than the length, 

 the hinge-line much shorter than the greatest width of the shell, the car- 

 dinal extremities rounded, the greatest width of the shell near its mid- 

 length. The dimensions of tw r o individuals are : length of pedicle valve 

 32 mm. and 35 mm., length of brachial valve 26 mm. and 28 mm., greatest 

 width 37 mm. and 33 mm., thickness 18 mm. and 24 mm., height of car- 

 dinal area 4.5 mm. and 4 mm. 



Pedicle valve more or less strongly convex, the convexity extending 

 out to the cardinal extremities, the surface curving abruptly from the 

 umbonal region to the cardinal margin and more gently to the antero- 

 lateral margins ; beak rather obtuse in the easts and more or less incurved ; 

 cardinal area of medium height, more or less arched, the lateral margins 

 defined but doubtless less defined in the casts than in the shell itself, 

 sloping from the beak to the cardinal extremities; lateral slopes each 

 marked by 25 or more depressed, bifurcating plications which become 

 faint or almost obsolete towards the cardinal extremities ; mesial sinus ill- 

 defined, shallow, rounded, sometimes scarcely more than a flattening of 

 the median portion of the valve, marked by 10 or 12 plications similar in 

 every respect to those of the lateral slopes. 



Brachial valve a little less convex than the pedicle, the greatest con- 

 vexity near or posterior to the middle, the surfacje curving rather ab- 

 ruptly from the umbonal region to the cardinal margin, and more gently 

 to the antero-lateral margins, sometimes a little compressed towards the 

 cardinal extremities; mesial fold low r , ill-defined, scarcely raised above 

 the general surface of the valve except sometimes towards the front, 



