120 J. S. GARDNER ON BRITISH CRETACEOUS NUCULIDA. 
4, British Cretaceous Nucvntipz. By J. Srarxie Garpner, Esq., 
F.G.8. (Read November 7, 1883.) 
[Puates JII.-V.] 
Tur Nucule and Lede, with their allies, are now generally separated 
from the Arcide, and in a paleontological point of view this separa- 
tion 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 Nuculide 
includes a number of genera and subgenera which it would be incon- 
venient to adopt here; for while probably every geologist would dis- 
tinguish a Nucula from an Arca 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 Creta- 
ceous fossils, of the internal characters of the shell being revealed. 
Were all adopted, the Nuculide from the Gault of Folkestone alone 
would require seven subgeneric names; but their distinctive cha- 
racters are relatively too trivial to be of paleontological value. 
The details of the hinge and the position of the muscular attach- 
ments 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 Lede and Nucule occasionally abound in the marine forma- 
tions 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. 
Teas the definition of new or imperfectly described species, a 
revision of the family is necessary, as confusion still exists in works 
on Cretaceous geology between the genera Nucula and Leda, owing 
to Sowerby, D’Orbigny, and other authors having described all the 
Cretaceous species as Nucula; and although a few Lede are now 
sometimes distinguished, these are still assigned indifferently to one 
genus or the other in lists of fossils. Yet the two genera are per- 
fectly definable, and both have been established for more than half 
a century. Again, several of the species are credited with a much 
wider range than they actually possess. Some species have clearly 
been founded on misapprehension, and a few others have crept into 
our lists which have not hitherto been met with in Great Britain, 
while the published illustrations of most of them are indifferently 
executed, and must be sought in expensive and often voluminous 
publications long since out of print, and the few descriptions of them 
in our language are meagre and imperfect. 
If we examine the Nuculide of a single Cretaceous horizon, as 
the Gault, and exclude all others, we see an assemblage of species 
which differ for the most part as widely as possible from each other. 
These seem to be forms whose ancestors were differentiated at least 
prior to the Jurassic period. But when the Nuculide from the 
