260 MISSISSIPPIAN BRACHIOPODA 



all members of the genus Dielasma, but the details of these structures can- 

 not be determined in the type specimen. 



Surface of the valves apparently nearly smooth except towards the 

 outer margin where there are several concentric lines of growth varying 

 in strength. The shell structure is finely and closely punctate. 



Remarks. This species resembles D. chouteauensis, but it is propor- 

 tionally broader, and has the transverse, muscle-bearing plate, between 

 the bases of the crural lamella?, attached to the floor of the valve through 

 the whole, or nearly the whole, of its length. 



Horizon. Fern Glen formation. 



DIELASMA OSCEOLENSIS n. sp. 

 Plate XXXIII, Figs. 1-3 



Description. Shell large, elongate-subovate in outline, the anterior 

 margin rounded, the greatest width near the mid-length. The dimensions 

 of a very complete but slightly distorted example, the holotype, are : 

 length of pedicle valve 52 mm., length of brachial valve 48 mm., greatest 

 width 32.4 mm., thickness 22.8 mm. 



Pedicle valve rather strongly convex, the greatest convexity posterior 

 to the middle, the surface arcuate from beak to front with the curvature 

 increasingly convex posteriorly, curving rather abruptly towards the 

 postero-lateral margins, becoming incurved to the cardinal extremities, 

 the curvature to the antero-lateral and anterior margins more gentle; 

 mesial sinus obsolete but the mesial portion of the valve is obscurely flat- 

 tened ; the beak prominent, pointed, strongly incurved, the deltidial plates 

 nearly hidden by the incurvature, the foramen very large, elongate-ovate, 

 encroaching wholly upon the umbo. Internally the dental plates are 

 strongly developed and elongate, reaching anteriorly from the beak nearly 

 one-fourth the length of the valve. 



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

 vexity posterior to the middle, the surface curving more abruptly to the 

 postero-lateral margins and somewhat gently to the antero-lateral and 

 anterior margins ; mesial portion of the valve obscurely flattened but not 

 differentiated from the general convexity ; the beak pointed and incurved 

 beneath that of the opposite valve. Internally the crural lamellae are 

 separate from the socket plates and diverge anteriorly from the beak, 

 extending about one-fifth the length of the valve ; entire brachidium not 

 observed. 



Surface of both valves marked only by concentric lines of growth which 

 are obscure posteriorly, becoming stronger and wrinkle-like towards the 

 front. Shell substance finely punctate. 



Remarks. This species is one of the largest in our faunas and resembles 

 some European specimens which have been identified as Terebratula hastata, 



