Sinus und Sattel " is the most noteworthy of the characteristics of this species. 

 In reality the dichotomous increase of the ribs takes place on the fold and in 

 the sinus in the Chinese examples also. But the lateral ribs are verj- regular 

 and simple in the latter, dichotomy being seldom met with. In this respect 

 the Chinese specimens now at hand do not appear quite coincident with the 

 species S/y. bisulcatits according to the circumscription of Scupin, who says. 

 " Die Zahl der Lateralrippen ist sehr verschieden, je nach dem ein mehr oder 

 weniger grosser Theil derselben eine Theilung erfahrt." The writer long 

 hesitated over this point. 



On the contrary Davidson says that these radial ribs " are simple, and 

 rarely bifurcated, but increase occasionally by intercalations at various dis- 

 tances from the beak." This species is separated from Sp. stiratiis, also an 

 allied form, in having " einfache, weniger zahlreiche, aber dafCir breitere, 

 gerundete Rippen, die sich gelegentlich teilen." De Konick's observation is 

 here very noteworthy : " In young specimens most of these folds are simple, 

 while in adults the two or three folds adjoining the ventral furrow and the 

 dorsal ridge are bifurcated. Scupin's figure, though not very clear, shows the 

 fact asserted by De Koninck, one or two of the ribs next to the median sinus 

 appearing to be dichotomous. In the Chinese specimens now under considera- 

 tion the same thing is observed in some of the larger examples. Thus the 

 latter are identified with Sp. bisulcatiis Sow. 



The shell is rather broader than long, the hinge-line representing the 

 maximum breadth ; outline, oval or subrhomboidal. Both valves are almost 

 equally convex and nearly equally curv^ed. Each valve has a strongly crooked 

 beak ; the two beaks almost touch each other. The ventral umbone is very 

 high above the hinge-line and pointed outwards. Beneath it is a high, tri- 

 angular but concav^e area with a delthyrium in the middle. The dorsal area 

 is not exposed, the umbonal expansion being very inconspicuous and hardly 

 over hanging the hinge-line. The sinus on the x'entral valve is already dis- 

 tinct at the tip of the beak, and gradually "increase in width anteriorly ; it is 

 moderately deep. Corresponding to it there is a distinct fold on the dorsal 

 valve, also traceable up to the b -ak. P3ach valve is ornamented with from 30 

 to 40, rounded, wide, simple radial ribs. Those on the told or in the .sinus are 



