494 MISSISSIPPIAN BRACHIOPODA 



obsolete sinus of the pedicle valve, and the absence of a mesial fold in 

 the brachial valve, in place of which the valve is flattened or slightly 

 sinuous. 



Horizon. Keokuk limestone. 



COMPOSITA OPPOSITA (White and Whitfield) 

 Plate LXXXII, Figs. 11-13 



1862. Rhynchonella opposita White and Whitfield, Proc. Boston Soc. Nat. 

 Hist, vol. 8, p. 294. 



Description. Shell very small, subtriangular in outline, broader than 

 long, the greatest width near the anterior margin, the postero-lateral 

 margins very long, nearly straight anteriorly, becoming slightly convex 

 posteriorly, meeting at the beak in an angle of about 100 degrees, the 

 anterior margin trisinuate, meeting the lateral margins in the acutely 

 angular antero-lateral extremities of the shell. The dimensions of the 

 holotype are : length of pedicle valve 5.5 mm., length of brachial valve 

 5 mm., greatest width 6 mm., thickness 3.2 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the cardinal extremities, less abruptly to the antero-lateral 

 margins and gently to the anterior margin; mesial sinus originating in 

 the umbonal region, shallow and rather narrow, rounded in the bottom, 

 not sharply defined laterally, upon each lateral slope another sinus 

 similar to the mesial one, but slightly shallower and narrower, originates 

 a little in front of the point of origin of the mesial sinus, continuing to 

 the anterior margin, these three depressions divide the entire surface 

 of the valve anteriorly into four rounded, subequal plications; the beak 

 incurved, pierced by a large foramen which encroaches upon the umbonal 

 portion of the valve; the delthyrium filled by the beak of the opposite 

 valve. 



Brachial valve subequally convex with the pedicle, the greatest depth 

 posterior to the middle, the surface curving abruptly to the cardinal 

 extremities, less abruptly to the lateral margins anteriorly, and more 

 gently to the anterior margin ; the surface of the valve anteriorly with 

 three shallow depressions entirely similar to those of the pedicle valve 

 and opposite .to them, the presence of the three depressions in each valve 

 producing the three emarginations in the anterior margin; the beak 

 strongly incurved beneath that of the opposite valve. 



Surface of both valves marked by more or less obscure, regular, con- 

 centric lines of growth. 



Remarks. The only example of this species which has come under the 

 observation of the writer is the holotype in the collection of the American 

 Museum of Natural History in New York. The original reference of the 

 species to the genus Rhynchonella is obviously incorrect, the specimen 



