221 



Dimensions : — 



Length, Width. Thickness. 



9 mm. lo mm. 5.5 mm. 



15 mm. 17 mm. 10 mm. 



Remarks : — The above diagnosis is based chiefly on a small specimen 

 which is the most complete among the examples at hand. The number of the 

 ribs is small because the shell itself is small. The shell is also less transverse 

 than those figured by Davidson or De Koninck, and this also would be 

 expected from the smallness of the specimen itself. " In the fry the shell is 

 at times somewhat triangular, the width being equal to the length, but with 

 growth the shape becomes more transverse, and rapidly increases in depth or 

 convexity, " remarks Davidson. De Koninck describs the number of the 

 ribs in the median sinus as varying from 4-7 in this species, and in reality in 

 none of the larger examples is the number below 4. But if the pictures of 

 the small examples are taken into account this number can be reduced to 

 three or even less (for instance in figures 17—20). Thus the Chinese speci- 

 mens are nothing but smaller or younger examples of RJiyiichonclIa plcurodon. 



Locality : — Huang-tu-pu, prov. Hu-nan. 



At this locality this species was collected together with Spirifcr bisulcatus 

 and AtJiyris royssii. 



Geological Age : — Of the three species of Brachiopods found in this 

 locality the present species is essentially confined in Europe to lower Carboni- 

 ferous rocks, although it has been reported from the upper Carboniferous at 

 Mjatschkovva. The other two have a very wide range of distribution both 

 vertical and horizontal, and reach up to the Permocarboniferous formations. 

 The only species therefore that determines the geological age is the present 

 one. As, however, the latter is not absolutely restricted to the lower Carboni- 

 ferous, the writer is much troubled as to the final determination of the sreo- 

 logical antiquity of the fauna. 



It is noteworthy that the three species from the very locality are 

 somewhat smaller than the same from the other parts of the world. 



