(l) IS Piihrfiiitoliif/iii Sinicri Ser. B 



At least one sp-eics n[ Slrophomenoid shell allied to .s'. inciirvaiii is common in 

 the upper .Alachiakon limestone. It is of medium size, moderately concavo-convex, witli 

 tlie hingeline forming the greatest \vidth of the shell. 



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

 s]^eeiraens the earliest stage of this valve_, now forming the heak, is more or less strongly 

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

 shoM-n at all. In consequence these suggest the characters of the brachial valve of a 

 I!tijinc>:qii{na. A S])ecimen of this kind however sliows a high hinge area, thus indicating 

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

 of the area is shown, but this is enough to sho\\' the triangular dtdthyrium partly covered 

 by a rather short deltidial plate. The contour of the \'alve 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 deju-essed or concave, as is often if not generallj" the case in the genus. 

 In some spi'cimens the center of the valve is most strongly elevated, the sides being 

 dei)ressed-convex. Such a sliell wlien small, has something of the aspect of a Dahannclla. 

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

 longitudinal contour shows tlie gr( atest convexity somewhat in front of the middle. In a 

 few specimens the Itjngitudinal contour is shghtly undulating, due to faint and ill-definecl 

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



Surface marked by rather' strong radiating strife A\inch increase in strength 

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

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

 striie appear in the widening intersjiaces. Thus the asj)ect of the surface is that of strong 

 stri;e with from tliree to five hner ones lietween each pair of coarser ones. Very fine 

 concentric lines cnncellate the stria- and mark the interspaces, where they are strongest. 

 They are very numerous and close-set. 



The A\ idth of an avi'rage specimen is 14 mm. the height 8 mm. or more. 



TIoRizox AND Loi'AfJTiKs: This is a common form in the massive dark-gray 

 calcilutytes of the upper Machiakou hmestone series at C'haokouchuang. As a rule the 

 shells are closely crowded, and so much a i)art of the rock, that it is ])ractically impossible 

 to prepare the s])eciinens, and oliservations are limited to the fracture exposures. Onlv 

 in rare cases, wheiv the rock has \\-eatliered upon the surface, is the shell visible in relief 

 and ])reparation with the needle point is possible. 



