2 /O 



shape of the scutum and tergum, the strongly marked crests for the depressor muscles, 

 extending beneath the basal margin of the tergum, are so characteristic that I could have 

 no doubt as to its specific nature. 



Yet, in some respects the "Siboga" specimen differs from Darwin's description. The 

 scutum and tergum were so intimately joined together that only a tracé of a suture could 

 be seen externally. By boiling these valves with caustic potash I easily separated them 

 from each other, but I think this would not have been the case if they were "completely 

 calcified together" as Darwin says they are. The inner basal edges of the parietes, moreover, 

 were not inflected inwards, as Darwin says is the case in all full-grown specimens. So far as 

 can be made out with the incomplete object at my disposal, it is probably full-grown, mea- 

 suring 1 1 mm. (the largest diameter of the basis), whilst the largest specimen that Darwin 

 saw was 0.35 of an inch (about 8 mm.) in basal diameter. 



The only specimen collected by the "Siboga" was taken at: 



Stat. 16. March 15/16, 1899. Lat. 6° 59' S., Long. U5°24'.7E. [Bay of Kankamaraan, S. coast 

 of Kangeang.] Depth 22 m. Bottom: mud. 



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