GIRTYELLA 275 



GlRTYELLA INDIANENSIS (Girty) 



Plate XXXIV, Figs. 1-24 



1891. Terebratula turgida Whitfield, Ann. N. Y. Acad. Sci., vol. 5, p. 586, 



pi. 13, figs. 21-22. 

 1895. Terebratula, turgida Whitfield, Geol. Surv. Ohio, vol. 7, p. 473, pi. 9, 



figs. 21-22. 

 1908. Uarttina indiamensis Girty, Proc. U. S. Nat. Mus. vol. 34, p. 293, 



pi. 19, figs. 6-15. 

 1911. Dielasma turgida Morse, Proc. Ohio Acad. Sci., vol. 5, p. 381, figs. 



12a-b. 

 1911. Girtyella indianensis "VVeller, Jour. Geol., vol. 19, p. 442, figs. 2a-i. 



Description. Shell small, longitudinally subovate, often approaching 

 subpentagonal in outline, the greatest width a little in front of the middle, 

 the anterior margin straightened or a little emarginate. The dimensions 

 of two complete examples are: length of pedicle valve 13 mm. and 11.7 

 mm., length of brachial valve 12 mm. and 10.4 mm., width 10.2 mm. and 

 9.5 mm., thickness 8 mm. and 6 mm. 



Pedicle valve most convex near or a little posterior to the middle, 

 the surface curving abruptly towards the postero-lateral margins and 

 becoming a little inflected to the cardinal extremities, curving less ab- 

 ruptly to the antero-lateral and anterior margins ; mesial sinus obsolete 

 in the umbonal region, rarely originating posterior to the middle, often 

 being confined to the anterior third or fourth of the valve, shallow, 

 rather narrow, rounded in the bottom and ill-defined laterally; the beak 

 rather prominent and projecting well beyond that of the opposite valve, 

 incurved, pierced by a large, longitudinally elliptical or subovate fora- 

 men which encroaches in its entirety upon the umbonal portion of the 

 valve ; the delthyrium and deltidial plates not visible externally because 

 of the incurvature of the beak. Internally the dental lamellae are well 

 developed and extend nearly one-fourth the length of the valve an- 

 teriorly from the beak with a moderate degree of divergence. 



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

 vexity at or near the middle, the surface curving rather abruptly to the 

 postero-lateral margins, arched from beak to front with the curvature 



C 



FIG. 33. A series of four cross-sections of the rostral portion of the brachial valve 

 of Girtyella indianensis (X 2%) from the Renault formation, showing the 

 median septum and concave hinge-plate. 



often a little more abrupt anteriorly; a mesial fold to correspond with 

 the sinus of the opposite valve is rarely developed, when present it is 

 narrow and obscure with a slight depression on either side and orig- 

 inates near the anterior margin, more commonly the median portion of 



