266 MISSISSIPPIAN BRACHIOPODA 



Description. Shell usually below medium size, elongate-ovate in outline, 

 the greatest width near or anterior to the middle, the anterior margin 

 rounded. The dimensions of a very perfect specimen are : length of ped- 

 icle valve 15.4 mm., length of brachial valve 13.8 mm., maximum width 

 10.2 mm., thickness 9 mm. 



Pedicle valve most convex posterior to the middle, arched along the 

 median line from beak to front with a convexity progressively increasing 

 posteriorly, the surface curving abruptly from the median line to the 

 postero-lateral margins and becoming inflected to the cardinal extremi- 

 ties, curving more gently to the antero-lateral margins ; mesial sinus obso- 

 lete ; beak prominent, incurved, pierced by a large, subcircular or subo- 

 vate foramen which encroaches wholly upon the umbonal region of the 

 valve, in contact with the delthyrium only at its apex ; delthyrium broadly 

 triangular, entirely closed, except around the border, by the beak of the 

 opposite valve and the deltidial plates which are invisible externally, be- 

 cause of the incurvature of the beak. Internally the dental lamellae are 

 well developed and extend anteriorly from the beak for nearly one-fourth 

 the total length of the valve. 



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

 vexity near or a little posterior to the middle, arched from beak to front 

 along the median line with the curvature a little greater posteriorly, the 

 surface curving rather abruptly from the median line to the postero- 

 lateral margins, becoming more gently curved anteriorly; mesial portion 

 of the valve not differentiated as fold or sinus ; the beak acutely pointed, 

 incurved beneath that of the opposite valve. Internally the socket plates 

 are separate from the crural plates and are not connected by a hinge- 

 plate; the crural plates originate near the apex of the valve about half 

 way between the median line and the lateral margins, gradually approach- 

 ing, relatively, the median line anteriorly, they are low and strongly in- 

 clined inward posteriorly, abruptly becoming very greatly increased in 

 height anteriorly and becoming more erect, posteriorly they are joined by 

 a concave, platform-like plate bearing the muscular scars, which is medi- 

 ally in contact with the inner surface of the valve leaving a pair of tri- 

 angular, finger-like cavities diverging from the beak, which are surround- 

 ed by the inner surfaces of the crural plates, the muscle plate and the 

 floor of the valve ; the brachidium is short, its anterior extremity falling 

 short of the middle of the valve. 



Surface of both valves nearly smooth or marked only by concentric 

 lines of growth which are sometimes strongly developed and rather closely 

 crowded. Shell structure finely punctate. 



Remarks. Various forms have been referred to this species in the liter- 

 ature. In volume eight of the New York Paleontology no less than four 

 distinct species belonging to two different genera are illustrated under 



