RHIPIDOMELLA 151 



those of the opposite valve and less deeply impressed, quadripartite and 

 subquadrate in outline ; the inner margin of the valve crenulate anter- 

 iorly and laterally. 



Surface of both valves marked by regular, uniform, depressed, rounded, 

 radiating costse, from two to three of which occupy one millimeter. The 

 shell substance is pierced by radiating canals or tubules of two sizes, 

 which apparently follow the direction of the costae, the larger ones open 

 at intervals along the summits of the costae while openings of the finer 

 ones cover the sides of the costae and the furrows, giving to the shell, 

 when a little weathered, the appearance of being punctate. 



Remarks. This species is characterized by the prominent umbo of the 

 pedicle valve, by the greatly elevated cardinal process of the brachial 

 valve and by the large size of the muscular scars in the pedicle valve. 

 In size it is about equal to R. oweni, but it is proportionally longer and 

 has a much more prominent umbo on the pedicle valve and nearly or 

 quite obsolete sinus in the brachial valve. It differs from R. jerseyensis 

 in its coarser surface costge, the different position of the cardinal area 

 of the pedicle valve, and in its much more prominent cardinal process. 

 The punctate appearance of the shell surface is commonly more con- 

 spicuous in this species than in any of the others here described, but this 

 may be due to the different condition of preservation. 



Horizon. Burlington limestone. 



Plate XXI, Figs. 1-4 



1882. Orthis Michelini (f) Hall, Rep. N. Y. State Geol. for 1882, pi. (6) 



36, figs. 19-21. 

 1392. Orthis (Rhipidomella) Oweni Hall and Clarke, Pal. N. Y., vol. 8, 



pt. 1, p. 342, pi. 6, figs. 19-21. 

 1897. Orthis (Rhipidomella} Oweni Hall, 14th Rep. N. Y. State Geol., 



p. 341, pi. 4, figs. 24-26. 



Description. Shell above medium size, lenticular in form, broader than 

 long, with the greatest width anterior to the middle, the hinge-line about 

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

 extremities rounded, the anterior margin nearly straight or somewhat 

 emarginate in its median portion, curving regularly into the lateral mar- 

 gins at each side. The dimensions of two individuals are : length of 

 pedicle valve 33 mm. and 27.5 mm., width 38 mm. and 32.3 mm., length 

 of hinge-line 17 mm. and 17 mm., thickness approximately 9 mm. and 

 7 mm. 



Pedicle valve most convex at a point about half way between the middle 

 and the beak, the surface curving more abruptly to the cardinal margin, 

 very slightly or not at all compressed towards the cardinal extremities, 



