334 MISSISSIPPIAN BRACHIOPODA 



been punctate, and it is not known to possess a median septum internally 

 in the pedicle valve. Neither does it possess the strong, concentric, 

 lamellose markings of the non-punctate Deltliyris. Under these circum- 

 stances, then, the species may be allowed to remain in the genus Spirifer 

 to which it was originally referred. 



Horizon. Lower Burlington limestone. 



SPIRIFER IMBREX Hall 



Plate XLIII, Fig. 17 



1858. Spirifer imbrex Hall, Geol. Iowa, vol. 1, pt. 2, p. 601, pi. 13, fig. 2. 

 1883. Spirifera imbrex Hall, Rep. N. Y. State Geol. for 1882, pi. (31) 56, 



figs. 11-12. 

 1894. Spirifer imbrex Hall and Clarke, Int. Study of Brach., pt. 2, pi. 27, 



fig. 1. 



1894. Spirifera imbrex Keyes, Mo. Geol. Surv., vol. 5, p. 80. 



1895. Spirifer imbrex Hall and Clarke, Pal. N. Y., vol. 8, pt. 2, pi. 31, 



figs. 11-12. 



Description. Shell above medium size, much broader than long, the 

 greatest width along the hinge-line. The dimensions of the holotype, a 

 brachial valve, are : length 27 mm., greatest width 52 mm., depth 7 mm. 



Brachial valve moderately convex, emarginate anteriorly in the holo- 

 type, greatest depth near the middle, the surface curving abruptly to the 

 cardinal margin on either side of the beak, becoming compressed towards 

 the cardinal extremities, with a gently convex curvature to the antero- 

 lateral margins ; mesial fold sharply defined to the beak where it is narrow, 

 gently convex and scarcely elevated above the lateral slopes, becoming 

 more angular' anteriorly but only moderately elevated; beak small, 

 incurved. Surface of the valve marked by from 22 to 24 angular 

 plications upon each lateral slope, and about 15 upon the mesial 

 fold, the larger plications measuring about 2 mm. from center to 

 center at the front of the shell, the plications of the lateral slopes fre- 

 quently bifurcate once in passing from the cardinal margin to the front, 

 all those of the fold originating from the subdivision of a single one at 

 the beak. Crossing the plications, in the holotype, are two very strong 

 concentric lines of growth anterior to the middle of the valve, and the 

 entire surface is covered by fine concentric, sublamellose markings, about 

 three of which occupy the space of 1 mm., and which are much more con- 

 spicuous anteriorly. 



Remarks. This species seems to be a rather unusual one in the Burling- 

 ton limestone fauna, the specimens which have usually been so identified 

 being examples of 8. incertus. Good examples of the pedicle valve of the 

 species have not been observed, in fact the holotype, which is a detached 

 brachial valve, is the only really good example which has been studied. 

 S. imbrex and 8. incertus are similar in size and form, but S. imbrex is 



