256 PALEONTOLOGY OF NEW YORK. 



tiou is usually lost, and only a median division by a slight longitudinal septum is 

 discernible. The entire area does not extend more than one-third the whole 

 length of the valve. Over the post-lateral slopes are numerous line, irregularly 

 ramifying genital sinuses. 



The brachial valve is considerably the less convex and is often flattened. 

 The beak is minute and usually obscured by the overlapping pedicle-valve. 

 The hinge-plate is large, fiat, triangular, sometimes thin, often thickened on its 

 posterior portion and resting on the bottom of the valve. It is separated from 

 the lateral shell-walls by narrow dental grooves widening at their extremities. 

 Normally this plate appears to have been perforated by a visceral foramen 

 entering at the underside and opening at or beneath the apex of the beak. 

 This perforation is however frequently filled by adventitious deposits though 

 traces of it are discernible in the oldest shells, and in casts of the interior the 

 filling of the tube is often preserved. Tlie median portion of the plate, lying 

 between two vertical supporting lamellse resting on the bottom of the valve, is 

 preserved in the earliest and simpler species, but in the larger and later forms 

 of the genus, is frequently resorbed, giving the plate the appearance of being 

 composed of subtriangular, discrete lateral halves. 



The crura are the continuation of the upper portions of the supporting septa. 

 Neither these nor the rest of the brachidium have been seen in the type species, 

 R. ovoides, of the Oriskany sandstone,* but in R. Marylandica, the crura are 

 broadened just beyond their base of attachment, and from their upper angles 

 are given off the jugal processes which are long, pointing upward and inward, 

 but not uniting. From the lower angles the descending arms take their origin, 

 following the curves of the valve, diverging for a short distance, thence abruptly 

 approaching, and uniting to form a broad elongate, acutely triangular plate, 

 which is not supported by a median septum, or otherwise connected with the 

 valve. From the middle of the posterior margin of the plate arises a small 

 rod-like process, which extends for a short distance upward toward the crura. 

 The entire length of the brachidium is nearly two-thirds that of the valve. 



*The tigufes of the brachial supiiorts ascribed to R. ovoides in the Twelfth Report on the Slate Cabinet 

 of Natural History, p. 41, repiesent the siiecies -S. Marylandica, which at that ilate had not been sejiarated 

 from R. ovoides. 



