British Cretaceous Nuculidæ

Abstract

T he Nuculæ and Ledæ with their allies, are now generally separated from the Arcidæ, and in a palæontological point of view this separation is amply justified; for their distinctive characters have persisted throughout many geological periods, and may be recognized to some extent now in the Silurian fauna. The family of the Nuculidæ includes a number of genera and subgenera which it would be inconvenient to adopt here; for while probably every geologist would distinguish a Nucula from an Area or from a Leda , even if imperfectly preserved, none but a specialist would be able to separate a Yoldia from a Leda , and only then in the event, not common among Cretaceous fossils, of the internal characters of the shell being revealed. Were all adopted, the Nuculidæ from the Gault of Folkestone alone would require seven subgeneric names; but their distinctive characters are relatively too trivial to be of palæontological value. The details of the hinge and the position of the muscular attachments do not appear to be of very great importance in this group and are not always visible in the fossils; they are therefore omitted from the abridged descriptions of the species. Both Ledæ and Nuculæ occasionally abound in the marine formations of the Cretaceous system, except in the White and the Red Chalk, whence they may have been dissolved out. The species do not differ greatly from those of the Jurassics, nor from those of the present day. Besides the definition of new or imperfectly described species,