492 MISSISSIPPIAN BRACHIOPODA 



Remarks. This species is characterized by the prominence of the umbo 

 of the brachial valve, the smoothness of the surface, the shortness of 

 the mesial sinus of the pedicle valve and the absence of a mesial fold 

 in the brachial valve, in place of which the surface of the valve is flat- 

 tened towards the front and usually somewhat depressed in a slight 

 mesial sinus close to the front margin. 



Horizon. Chester group. 



COMPOSITA PENTAGONIA n. Sp. 



Plate LXXXII, Figs. 52-57 



Description. Shell below medium size, broader than long, subpentag- 

 onal in outline, the postero-lateral margins longer than the other sides 

 of the pentagon and meeting at the beak in an angle of about 105 degrees, 

 laterally the margins are rounded, becoming a little concave antero- 

 laterally, the anterior margin rather short and einarginate. The dimen- 

 sions of two very perfect internal casts are : length of pedicle valve 15.5 

 mm. and 14.8 mm., length of brachial valve 14 mm. and 12.8 mm., greatest 

 width 16.3 mm. and 16.5 mm., thickness 11 mm. and 10.3 mm. 



Pedicle valve convex in the umbonal region, the surface curving ab- 

 ruptly as it approaches the postero-lateral margins, and in the internal 

 casts a little incurved to the cardinal extremities, towards the antero- 

 lateral margins the surface is regularly convex and along the median 

 line from beak to front it describes nearly a semicircle; mesial sinus orig- 

 inating near the middle of the valve, shallow, of moderate width, and 

 rounded in the bottom, laterally its surface rounds regularly into the 

 lateral slopes of the valve so it is not sharply defined ; the beak strongly 

 incurved, the foramen apparently rather small, the delthyrium filled 

 with the beak of the opposite valve. Internally the dental lamellae are 

 rather strong and reach anteriorly from the beak from one-fifth to one- 

 fourth the length of the valve. 



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

 vexity near or a little posterior to the middle, the surface curving more 

 abruptly to the postero-lateral margins, more gently to the antero-lateral 

 and anterior margins; mesial fold originating near the middle of the valve, 

 low, flattened on top and towards the front depressed in a slight longi- 

 tudinal sinus producing the emargination of the anterior margin of the 

 shell, laterally the fold is bounded by slight depressions more notice- 

 able towards the front, which produce the concavity or emargination of 

 the antero-lateral margins of the shell; the beak strongly incurved be- 

 neath that of the opposite valve. 



Surface of both valves, as seen in the internal casts, marked only by 

 rather strong, somewhat irregular, concentric lines of growth towards 

 the front. 



