Of this shell I have found but one specimen, which, how- ever, is in good preservation, and is probably identical with the British recent species.

It is however rather more trans- verse, the posterior or acuminated side being a little more produced, thereby removing the umbo farther from the centre, but from the examination of only a single specimen I should not venture to regard these distinctions as specific.

I found it myself in undisturbed red crag, three feet beneath the super- incumbent sand.

I have given the above as synonymes, presuming all to refer to the same species, although there are some slight dif- ferences which require notice.

The crag shell appears to be smaller than any of those quoted, and among fifty specimens that I possess, not one is more than two-thirds the size of the Nucula given me by Mr Smith, and which was obtained by him in the deposit exposed by the cutting for the Greenock railway.