RHIPIDOMELLA 157 



in the anterior portion of the same valve, as well as in the absence of 

 the mtesial sinus of the brachial valve. From R. diibia, it differs in its 

 longer hinge-line and larger cardinal area, and in its more quadrangular 

 outline. 

 Horizon. Upper Kinderhook. 



RHIPIDOMELLA JERSEYENSIS n. sp. 

 Plate XX, Figs. 36-43 



1909. RhipidGmella michelinia Weller, Bull. Geol. Soc. Am., vol. 20, p. 295, 

 pi. 12, figs. 8-10. 



Description. Shell small, wider than long, broadly subovate, frequently 

 emarginate in front, the greatest width anterior to the middle, the hinge- 

 line less than one-half the greatest width of the shell, the cardinal ex- 

 tremities rounded. The dimensions of a nearly complete specimen of av- 

 erage size are : length of pedicle valve 14 mm., length of brachial valve 13 

 mm., maximum width 15 mm., length of hinge-line 6.5 mm., thickness 

 6.7 mm. 



Pedicle valve most convex posterior to the middle, the surface curving 

 abruptly to the cardinal margin and more gently to the lateral and ante- 

 rior margins, the cardinal extremities usually somewhat compressed; the 

 median portion of the valve marked by an obscure, broadly rounded fold 

 which is not defined laterally and which frequently becomes flattened out 

 and obsolete anteriorly; the beak small, extending posteriorly only a 

 short distance beyond the hinge-line ; the cardinal area small, nearly flat 

 or gently concave, its lateral margins sharply defined, sloping posteriorly 

 at an angle of 30 degrees or less to the plane of the valve ; the delthyrium 

 large, broadly triangular, its width along the base being about one-third 

 the total length of the hinge-line. Internally the cardinal teeth are of 

 moderate strength and rather widely divergent ; the muscular scars are of 

 moderate size, extending anteriorly less than one-half the length of the 

 shell from the beak, and are only slightly depressed below the general 

 internal surface of the valve; the lateral and anterior margins crenulate 

 internally. 



Brachial valve nearly equally convex with the pedicle, its greatest con- 

 'vexity near or posterior to the middle, the surface sloping with a gently 

 convex curvature in all directions, the cardinal extremities often a little 

 compressed; the median portion of the valve marked by a distinct but 

 shallow, rounded, median sinus, originating in the umbonal region and 

 becoming stronger anteriorly, not sharply defined laterally; the beak 

 small, not incurved; the cardinal area almost obsolete, lying in the plane 

 of the valve. Internally the dental plates are rather prominent and 

 widely divergent ; the cardinal process is very small, rising slightly from 

 the posterior extremity of a rather broad, low, median ridge which ex- 

 tends anteriorly along the floor of the valve to the anterior margin of the 



