. RHIPIDOMELLA 153 



about two or three of which occupy the space of one millimeter at the 

 outer margin of a mature specimen of average size. Regular, concentric 

 growth lines are present upon both valves, sometimes rather uniformly 

 distributed but usually more crowded towards the front ; much finer con- 

 centric lines may be also detected upon some shells. The shell substance 

 is pierced by radiating canals or tubules of two sizes, following the 

 direction of the surface costas, the larger ones apparently occupy the 

 position of the median lines of the costse, along the summits of which 

 openings into the canals are present at irregular intervals, the finer canals 

 occupy the slopes of the costae and the intervening furrows, and upon 

 slightly weathered specimens their openings are so numerous as to give to 

 the shell a decided punctate appearance. 



Remarks. As, commonly preserved the shells of this species are crushed 

 and often distorted, but a sufficient number of undistorted examples, 

 mostly detached valves, have come under observation to show all the 

 essential features of the species. The species is especially characterized 

 by its proportionally broader form and smaller muscular scars of the 

 pedicle valve than is usual in the genus, and by the obscure and ill- 

 defined mesial fold of the pedicle valve which is bordered on either side 

 by an obscure rounded furrow. This latter character is not of sufficient 

 strength to be preserved in the crushed examples, but in the undistorted 

 valves it seems always to be present. The mesial sinus of the brachial 

 valve is much stronger than the two furrows of the pedicle valve, and is 

 a much more conspicuous feature than in R. burlingtonensis, the species 

 which this one perhaps most closely resembles. 



Horizon. Kinderhook (New Providence shale, Kentucky). 



RHIPIDOMELLA DIMINUTIVA RowJry 

 Plate XX, Figs. 9-18, 30-35 



1900. Rhipidomella diminutiva Rowley, Am. Geol., vol. 25, p. 261, pi. 5, 

 figs. 41-43. 



Description. Shell usually a little below medium size, subovate in out- 

 line, a little longer than wide or rarely with the length and breadth nearly 

 equal, the greatest width anterior to the middle, the hinge-line from one- 

 third to one-half the total width of the shell, the cardinal extremities 

 rounded. The dimensions of three specimens are: length of pedicle 

 valve 19 mm., 20 mm., and 19.3 mm.; length of brachial valve 18 mm., 

 19 mm., and 18.9 mm.; greatest width 17.4 mm., 18.8, and 19 mm.; thick- 

 ness 8.8 mm., 10.2 mm., and 9 mm. ; length of hinge-line 6.5 mm., 7.5 mm., 

 and 6.9 mm. 



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

 abruptly to the cardinal margin, more gently to the lateral margins and 

 still more gently to the anterior margin, slightly compressed towards the 

 cardinal extremities ; the median portion of the valve obscurely flattened 



