J. S. GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 123 
classification of the Chalk into zones of age has been greatly favoured 
by the fact that each zone of depth seems to contain a fauna some- 
what special to it, and that these faunas in the deeper sea-zones 
were so slightly modified with lapse of time that hardly any spe- 
cific distinction is visible, and the faunas of the newest beds or 
outliers of the Chalk with flints can be almost correlated palseon- 
tologically with the oldest; though even in these a careful study will 
reveal progressive change*. A modified Cretaceous fauna survived 
in the newest members of that series in America, into what were, 
perhaps, almost the older Eocene times in Europe. Much of the 
true Chalk fauna still exists in the moderate depths of the Atlantic ; 
and it is doubtful, save for the extinction of some of the most cha- 
racteristic groups, whether an upheaval would not reveal beds which 
not many years since might, too, have been classified as Cretaceous. 
In the more littoral outlying Greensand Beds, where Gastropods are 
preserved, we have, however, a successive and rapid modification 
towards an Hocene fauna; and upon the present hypothesis not only 
is this fact explained, but the far more anomalous one, that beds 
hitherto supposed to be of the same age were preceded and accom- 
panied in England by a purely Jurassic flora, while in Rhenish 
Prussia and America they were preceded by a flora with a prepon- 
derance of dicotyledons of a singularly Tertiary aspect. These are 
problems not perhaps immediately connected with the subject in 
hand, and an apology is needed for so long a digression; yet, as a 
scratched pebble may lead to the discovery of an ancient glaciation, 
so the apparently most uninteresting group of fossils may furnish a 
clue to the mode of deposition of a great formation. 
Nucvia, Lamarck, 1799. 
This genus, exclusive of Leda, comprises small inequilateral shells, 
nacreous internally, with a hinge composed of numerous interlocking 
teeth and a cartilage-pit. They are more or less truncated in front, 
with a flattened region termed the lunule; possess prominent um- 
bones, and are tumid or seldom compressed. The genus is restricted by 
some writers to those species which possess a denticulated margin?. 
The Cretaceous species, exclusive of Leda, are separable into two 
great divisions, the ovate and the subangular forms, and these may 
again besubdivided into smooth-shelled and sculptured-shelled groups. 
The two principal divisions are of great antiquity, their separation 
dating back in all probability to Paleozoic times. In the ovate 
division, which I refer to as the Ovate, the apices of the umbos are 
* Mosasawrus takes the place of the older Cretaceous Reptilia; Baculites and 
Aturia appear among Cephalopods; the Belemnitella of our Upper Chalk 
characterizes alike the Chalk of Ireland, Aix, and Denmark. Scaphites and 
Ammonites remain; but in what direction the newest forms had developed 
before they became extinct is still unknown. These with Znoceramus and Pleuro- 
tomaria, and some Brachiopods, form the latest Cretaceous groups. 
t Such a classification would, in this case, be extremely inconvenient ; ‘for the 
entire division of ‘‘ Ovate” would have to be removed. 
