l'i) 18 PaliKontologia Sinica Ser. B 



At least one species of Strophomenoid shell allied, to 8. incurvala is common in 

 the upper Machiakou limestone. It is of medium size, moderately concavo-convex, with 

 the hingeline forming the greatest width of the shell. 



Pedicle valve gently concave with a well developed hinge area. In some of the 

 specimens the earliest stage of this valve, noAV forming the Leak, is more or less strongly 

 convex but this is usually a very small apical portion, and in some shells this is not 

 shown at all. In consequence these suggest the characters of the brachial valve of a 

 Rafinesquina. A specimen of this kind however shows a high hinge area, thus indicating 

 that it is the pedicle valve, and that the generic reference is to Strophomena. Only half 

 of the area is shown, but this is enough to show the triangular delthyrium partly covered 

 by a rather short deltidial plate. The contour of the valve varies in different specimens 

 from nearly flat to pronouncedly concave, the greatest concavity being in the anterior 

 third. 



Brachial valve strongly convex. No specimen has been observed in which the 

 apical portion is depressed or concave, as is often if not generally the case in the genus. 

 In some specimens the center of the valve is most strongly elevated, the sides being 

 depressed-convex. Such a shell when small, has something of the aspect of a Dalmanella. 

 In other specimens, the contour of the valve is regularly arched transversely, while the 

 longitudinal contour shows the greatest convexity somewhat in front of the middle. In a 

 few specimens the longitudinal contour is slightly undulating, due to faint and ill-defined 

 concentric wrinklings, while near the front, the valve tends downward rather abruptly. 



Surface marked by rather strong radiating stria? which increase in strength 

 forward, and are multiplied by the intercalation of other stria;. These are at first much 

 finer, but also become strengthened towards the front, while at the same time other fine 

 stride appear in the widening interspaces. Thus the aspect of the surface is that of strong 

 strise with from three to five finer ones between each pair of coarser ones. Very fine 

 concentric lines cancellate the striae and mark the interspaces, where they are strongest. 

 They are very numerous and close-set. 



The width of an average specimen is 14 mm. the height 8 rnm. or more. 



HORIZON AND LOCALITIES: This is a common form in the massive dark-gray 

 calcilutytes of the upper Machiakou limestone series at Chaokonchuang. As a rule the 

 shells are closely crowded, and so much a part of the rock, that it is practically impossible 

 to prepare the specimens, and observations arc limited to the fracture exposures. Only 

 in rare cases, where the rock has weathered upon the surface, is the shell visible in relief 

 and preparation with the needle point is possible. 



