Rathbun.] 252 [December 16, 



one-third the length of the shell from the anterior extremity, and in 

 the interior moulds, in which state alone they have been obtained, 

 they are acute at the apex and curve strongly inward and moder- 

 ately forward. They closely approach one another. 



The valves are most prominent just above the middle, and arch 

 strongly from the beaks to the ventral margin. The curvature 

 along the antero-posterior axis is strong and generally regular ; some- 

 times the slope is more abrupt posteriorly. 



The surface is ornamented with prominent narrow plications, sepa- 

 rated by broad interspaces. The plications are very fine and high 

 at the beaks, and gradually increase in size toward the margins. 

 The interspaces are profound, flattened or slightly rounded in the 

 bottom, and two to three times as wide as the plications. At the base 

 of each plication, on either side, runs a fine thread-like line, which 

 seems to begin near the beak and extends to the margin, increasing 

 slightly in size. Yery fine and numerous concentric lines cross the 

 valves, and on the plications are grouped together so as to form reg- 

 ular bead-like prominences. On worn specimens the separate char- 

 acter of the beads is lost, and the plications appear rounded and 

 thread-like on the summit. In the interior of the valves the plica- 

 tions are very apparent, and they have left their imprint upon the 

 interna] moulds, as low rounded plications and interspaces of about 

 equal width, which die out near the beak. 



The inner margin of the valves is crenulated, while just within 

 the margin there is sometimes a smooth band or area of varying 

 width. The anterior adductor muscular impression is small and 

 slightly excavated. The imprint of the very small pedal muscle, ly- 

 ing above the anterior adductor, is entirely separated from it. 



The general characters of this form are those of Cardita, and the 

 hinge characters, so far as they are preserved or exposed, seem 

 also to agree with those of that genus. There is ah elongate poste- 

 rior lateral tooth, and above this a fine linear prominence. The pli- 

 cations in the genus Cardita do not generally show as prominently in 

 the interior of the shell as happens in this form. A specimen of 

 good size measured: length, about 23 mm.; height, 23 mm.; and 

 depth of both valves, 1 7 m;n. ; but the shell is usually more flattened 

 than in this case. 



Abundant in the cretaceous limestone bed at Sao Jose, Prov. of 

 Pernambuco, Brazil. 



