SPIKIFER 353 



extension, it is marked by a median plication which originates near the 

 beak, or at least in the umbonal region, and continues to the anterior 

 margin with gradually increasing strength, sometimes this median plica- 

 tion is the only one present in the sinus, but not infrequently there is a 

 single lateral plication on each side which originates from the inner 

 margins of the bounding plications, making three in all, the median 

 plication is about equal in strength to the larger of the plications on the 

 lateral slopes of the valve, the other two, when present, being somewhat 

 weaker. 



Brachial valve less convex than the pedicle, its greatest convexity 

 posterior to the middle, the beak sharply pointed, extending a little beyond 

 the hinge-line and slightly incurved over the narrow cardinal area, the 

 surface of the valve sloping more abruptly to the cardinal margin, slightly 

 or not at all compressed towards the cardinal extremities; mesial fold 

 defined to the beak, but slightly elevated posteriorly, attaining a moderate 

 height towards the anterior margin, where it has a rounded contour, 

 marked by a single, shallow, rounded, median furrow, and sometimes by a 

 still fainter one upon each lateral slope; the lateral slopes are marked 

 by plications which are similar in size and number to those of the opposite 

 valve. 



The more minute surface markings are not well preserved upon the 

 specimens, but there are fine concentric lines of growth towards the 

 anterior margin, and occasionally stronger ones. 



Remarks. 8. indianensis resembles S. keokuk in its type of surface 

 markings, but it can be distinguished at once from that species by its 

 elongate form. This elongation of the shell is rather an unusual form 

 among the species of Spirifer in our Mississippian faunas, and the species 

 may be recognized by this character alone, as there is no other closely 

 allied species, with such a form. 



Horizon. Keokuk (Harrodsburg limestone of Indiana). 



SPIRIFER ROSTELLATUS Hall 

 Plate XLVITI, Figs. 6-16 



1858. Spirifer rostellatus Hall, Geol. Iowa, vol. 1, pt. 2, p. 641, pi. 20, 

 figs. 2a-c. 



1894. Spirifera rostellata Keyes, Mo. Geol. Surv., vol. 5, p. 82. 



1895. Spirifer rostellatus Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 32, 



%. 5. 



Description. Shell of medium size, subovate to subelliptical in outline, 

 varying from longer than wide to wider than long, the greatest width 

 near the mid-length of the shell, the hinge-line much shorter than the 

 greatest width, the cardinal extremities rounded. The dimensions of a 

 complete individual, the holotype of the species, are : length 27.5 mm., 

 12 



