FOSSILS OP THE CLINTON GROUP. 579 



Ortkis {Plafystrophia ) biforata, Schlotheim. 



( Plate 25, Figs. 7, 8' ) 



Brachial valve with a strong median fold corresponding to a deep 

 sinus in the opposite valve. Both valves very convex, marked by strong 

 radiating plications, traversed by fine striae of growth, more frequent 

 towards the margins ol older specimens, and further ornamented by 

 minute granules, arranged in more or less regular close, intersecting 

 rows. One of the diameters of the quincuncial figures thus formed is 

 often fairly parallel to the zigzaging striae of growth where these 

 traverse the sides of the plications. Hinge area in both valves fairly 

 erect, incurved, leaving perhaps less of the area exposed to view in a 

 perfect shell than is true of many lower Silurian varieties of this species. 

 The area is marked by straight parallel horizontal striae of growth, cor- 

 responding to, and often connecting with the striae of growth on the' ex- 

 terior surface of the valves. Vertical to these are still finer parallel 

 striations, crossing the area in its shorter diameter, and visible, only un- 

 der a lense. Delthyrium open in both valves, the hinge area on either 

 side slightly elevated into a sort of bordering ridge. 



In the brachial valve this ridge after crossing a slight groove is con- 

 tinued in the acute angled outline of the crural plate where the latter 

 borders the delthyrium. The plate is strongly recurved, so that the 

 upper and broader side of its somewhat triangular acute tip passes be- 

 neath the corresponding tooth of the pedicle valve along its inner side, 

 . a deep notch in the outer angle between the plate and the hinge area 

 serving as the resting place for the tooth of the pedicle valve. The 

 acute, strongly recurved tips of the crural plates of the brachial valve 

 are rarely well preserved. The inner margin of the tooth of the pedicle 

 valve is also evidently in line with the raised margin of the hinge area 

 bordering the delthyrium, and it likewise has a deep notch along its , 

 outer angle, to receive the adjoining border of the hinge area of the 

 brachial valve. In specimens with strongly incurved beaks at times 

 only two harrow, slits communicating with the space between the hinge 

 areas remain for the exit of any pedicle. 



In the brachial valve the cardinal process is a low linear ridge. 

 This ridge in well preserved specimens is however often bordered on 

 either side by two similar ridges, which may be of the same size through- 

 out, may widen anteriorly, or may be replaced anteriorly by another 

 pair of ridges. 



The valves are thin, readily showing the radiating plications in- 

 eriorly, except in the, vicinity of the muscular area; at the latter place 

 theshellis considerably thickened. The muscular area of the brachial valve 

 is quadruplicate, the posterior adductor scars being sunk into the thickened 

 shell; they are bounded laterally by low, broad ridges, often not well 

 defined ; their limits anteriorly are indicated by the line at which the 



