278 MISSISSIPPIAN BRACHIOPODA 



GlRTYELLA BREVILOBATA Swallow 



Plate XXXIV, Figs. 37-41 



1863. T erebratula brevilobata Swallow, Trans. St. Louis Acad. Sci., vol. 

 2, p. 84. 



1894. Dielasma turgida Hall and Clarke, Int. to Study of Brach., pt. 2, 



pi. 53, fig. 10 (not figs. 11-12). 



1895. Dielasma turgida Hall and Clarke, Pal. X. Y., vol. 8, pt. 2, pi. 81, 



figs. 7-8 (not figs. 1-6). 

 1897. Dielasma turgida var. elongata Weller, Trans. N .Y. Acad. Sci., vol. 



16, p. 260, pi. 18, fig. 6. 

 1911. Harttina brevilobata Girty, Bull. U. S. Geol. Survey No. 439, p. 65, 



pi. 2, fig. 12. 

 1911. Girtyella brevilobata Weller, Jour. Geol., vol. 19, p. 443. 



Description. Shell small, longer than wide, subpentagonal or subovate 

 in outline, the greatest width near the mid-length, the anterior margin 

 truncated or more usually emarginate. The dimensions of a nearly per- 

 fect example are: length of pedicle valve 12.5 mm., length of brachial 

 valve 11.2 mm., width 10 mm., thickness 7.5 mm. 



Pedicle valve most convex posterior to or near the middle, the surface 

 curving abruptly to the antero-lateral margins, becoming inflected to the 

 cardinal margins, curving more gently to the antero-lateral and anterior 

 margins; mesial sinus usually well developed, originating near the um- 

 bonal region, narrow, rounded in the bottom, becoming rather deep ante- 

 riorly; the beak rather pointed, projecting notably beyond that of the 

 opposite valve, incurved, pierced by a subovate or subelliptical foramen 

 which encroaches entirely upon the umbonal region of the valve ; the del- 

 thyrium and deltidial plates hidden by the incurvature of the beak. In- 

 ternally the dental lamellae are well developed and extend anteriorly with 

 moderate divergence for one-fourth or a little more than one-fourth the 

 length of the valve. 



Brachial valve subequally convex with the pedicle, the greatest convex- 

 ity near the middle, the surface arched from beak to front with the pos- 

 terior slope a little more abrupt, curving abruptly to the lateral margins ; 

 at a point from one-third to one-fourth the length of the valve from the 

 anterior margin a rather broad mesial sinus or depression originates, 

 along the median line of which is a narrow, rounded fold, somewhat nar- 

 rower than the sinus of the opposite valve, giving to the valve a distinctly 

 trilobate contour with the median lobe smaller than the two outer ones 

 and a little depressed between them ; the beak pointed, strongly incurved 

 beneath that of the opposite valve. Internally the socket plates are con- 

 nected by a slightly concave hinge-plate which is supported by a median 

 septum. 



Surface of both valves smooth and unornaraented except by concentric 

 lines of growth which vary in the strength of their development in differ- 

 ent individuals. Shell structure finely punctate. 



