332 MISSISSIPPIAN BRACHIOPODA 



24.3 mm., greatest width along the hinge-line 73 mm., thickness 24.5 mm., 

 height of pedicle cardinal area 6.2 mm., height of brachial cardinal area 

 2 mm., width of sinus in front 12 mm. 



Pedicle valve with its greatest depth posterior to the middle, the sur- 

 face curving rather abruptly from the umbonal region to the cardinal 

 margin on each side of the beak, the slope to the cardinal extremities 

 nearly straight or slightly concave, that to the anterior margin gently 

 convex, towards the cardinal extremities the surface is somewhat com- 

 pressed in a direction subparallel to the cardinal area ; mesial sinus rather 

 narrow and more or less well denned, originating at the beak, usually 

 shallow but sometimes of moderate depth, rounded in the bottom ; the 

 beak small, pointed, rather strongly incurved ; the cardinal area of mod- 

 erate height, concave with the curvature increasing towards the beak, the 

 lateral margins sharply defined and subparallel with the hinge-line well 

 out towards the cardinal extremities, the inferior flatter portion lying at 

 an angle of about 120 degrees to the plane of the valve ; the delthyrium 

 large, broader than high. Each lateral slope marked by from 25 to 30 

 simple, depressed, rounded plications, separated by narrow, angular fur- 

 rows, originating at or near the cardinal margin, those towards the car- 

 dinal extremities becoming very faint ; the mesial sinus marked by a 

 median plication originating in the umbonal region and by two or three 

 lateral ones on each side which originate from the inner margins of the 

 bounding plications. 



Brachial valve subequally or less convex than the pedicle, its greatest 

 depth posterior to the middle, the surface curving rather abruptly to the 

 cardinal margin and more gently to the anterior margin, from the point of 

 greatest depth to the cardinal extremities the slope is gently concave, the 

 umbo slightly protuberant beyond the cardinal margin ; the mesial fold is 

 sharply defined, gently convex or flattened on top, originating at the beak 

 and only moderately elevated above the general surface of the valve ; a 

 distinct cardinal area with sharply defined lateral margins subparallel 

 with the hinge-line is present, it is gently concave and lies nearly in the 

 plane of the valve ; the beak small, strongly incurved. Each lateral slope 

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

 opposite valve, the plications of the mesial fold also entirely similar to 

 those of the sinus and all originating from the subdivision of the single 

 plication at the point of origin. 



Surface of both valves marked by fine, crowded, very regular, concen- 

 tric, lamellose lines which are more closely crowded at intervals to form 

 more or less conspicuous lines of growth. 



Remarks. This species is one of the less common Spirifers in the fauna 

 of the Burlington limestone, the brachial valve especially being rare. It 

 may be easily recognized by its elongate hinge-line and acute cardinal ex- 

 tremities. It differs from the associated S. incertus, which has similar 



