306 MISSISSIPPIAN BRACHIOPODA 



Pedicle valve strongly convex, the surface sloping with a gently convex 

 curvature from the umbonal region to the cardinal extremities, and 

 curving more abruptly to the anterior margin ; mesial sinus originating at 

 the beak, sharply defined, rounded in the bottom, becoming rather deep 

 anteriorly and produced in a rounded, lingual extension in front, of 

 greater or less length ; beak moderately incurved, remote from that of the 

 opposite valve ; cardinal area high, concave, with the concavity increasing 

 towards the beak, the lateral margins sharply defined, sloping from the 

 beak to the cardinal extremities with an increasingly convex curvature 

 distally, the delthyrium higher than wide. Surface of valve marked by 

 strong, simple, rounded or subangular plications which originate at the 

 beak or along the cardinal margin, the mesial sinus is marked by a single 

 faint plication along its median line which may be entirely obsolete in 

 the internal casts ; each lateral slope marked by from six to eight plica- 

 tions which become successively fainter towards the cardinal extremities ; 

 concentric lines of growth mark the valve which not infrequently become 

 rather strong towards the margin. Internally the dental lamellae are well 

 developed and sometimes extend anteriorly for one-fourth the length of 

 the valve from the beak, following the lines of the first intercostal furrows 

 each side of the sinus; between the dental lamellae is a strong median 

 septum which extends farther anteriorly than the dental lamellae, some- 

 times nearly to the middle of the valve. 



Brachial valve somewhat less convex than the pedicle, its greatest 

 convexity near or in front of the middle, the surface curving more 

 abruptly to the cardinal margin towards the beak, usually a little de- 

 pressed towards the cardinal extremities; the mesial fold well defined 

 by furrows which are wider and deeper than those between the plications, 

 becoming rather highly elevated towards the front, flattened on top with a 

 median furrow which is more conspicuous than the median rib of the 

 sinus of the opposite valve; beak small, moderately incurved. Surface 

 marked by plications similar in number and form to those of the opposite 

 valve, and by similar concentric lines of growth. Internally a slight 

 median ridge extends anteriorly from the beak to near the middle of the 

 valve. 



Remarks. In general formi and size this species resembles Cyrtina 

 neogenes, and the two shells have sometimes been confused. In the con- 

 dition of internal casts in chert, the only condition in which either of 

 these species have been certainly recognized, D. similis can always be rec- 

 ognized by the presence of three distinct slits diverging anteriorly from 

 the beak of the pedicle valve, representing the dental lamellae and the 

 median septum, while in C. neogenes a single median slit is present. The 

 plications of D. similis, also, are usually somewhat more subangular than 

 those of C. neogenes, and by this character the brachial valves of the two 



