RHIPIDOMELLA 159 



10.2 mm., length of brachial valve 14.6 mm. and 9.9 mm., maximum width 

 16 mm. and 10 mm., length of hinge line 5 mm. and 3.2 mm., thickness 6.7 

 mm. and 4.8 mm. 



Pedicle valve most convex posterior to the middle near the umbonal 

 region, the surface curving abruptly to the cardinal margin and gently 

 to the lateral and anterior margins, usually a little compressed towards 

 the cardinal extremities; the mesial portion of the valve usually not dif- 

 ferentiated from the general curvature of the surface ; the beak small, a 

 little incurved; the cardinal area very small, sloping posteriorly in an 

 angle not exceeding 25 degrees to the plane of the valve, sometimes ap- 

 parently lying almost in the plane of the valve; the delthyrium large, 

 broadly triangular, its width along the base more than one-third the total 

 length of the hinge-line. Internally the hinge-teeth are of moderate size 

 and are widely divergent; the characters of the muscular scars have not 

 been satisfactorily determined, but they do not extend anteriorly beyond 

 the middle of the valve ; the inner margins are crenulate anteriorly and 

 laterally. 



Brachial valve about equally convex with the pedicle, the greatest con- 

 vexity near the middle, the surface curving to the margins rather gently 

 in all directions, the cardinal extremities sometimes a little compressed ; 

 the mesial portion of the valve either not differentiated from the general 

 curvature of the surface or depressed in a slight mesial sinus which is 

 narrow, shallow and ill defined laterally; the beak shorter and more ob- 

 tusely pointed than that of the opposite valve, not incurved ; the cardinal 

 area very small, lying in nearly the plane of the valve. Internally the 

 cardinal process is large and strong, its posterior face transversely convex 

 and sloping posteriorly from the summit at an angle of about 45 degrees 

 to the plane of the valve, anteriorly it is continued along the median line 

 of the inner surface of the valve as a conspicuous, broad, rounded ridge 

 which reaches to about the middle of the valve; the socket plates flank 

 the cardinal process on either side, and are abruptly bent to an anterior 

 direction, and for the greater part of their length they are parallel with 

 each other and with the median line of the valve ; the characters of the 

 muscular scars have not been clearly determined. 



The surface of each valve is marked by exceedingly fine, nearly uni- 

 form, radiating costae which increase by bifurcation and intercalation, 

 about four or five occupying the space of one millimeter. The costae are 

 crossed by concentric lines of growth of moderate strength, varying in 

 number and distribution upon different individuals. The shell substance 

 is apparently perforated by radiating canals or tubules following the di- 

 rection of the costse, which open at intervals along the summits of the 

 costse ; the presence of finer tubules in the intervening spaces is not clearly 

 shown in any of the examples studied. 



