J. §S. GARDNER ON BRITISH CRETACEOUS NUCULIDA. 4a 
sudden appearance and disappearance, afford data by which, in 
England at least, horizons might be unerringly recognized; while 
the rest of the Vucule and Lede, from their great abundance, are 
also not without a certain value in this respect when carefully 
studied. The accompanying lists will at all events show clearly 
what species may be looked for in each of our British Cretaceous 
series, and will render the, at present, exceedingly difficult task of 
determining them a comparatively light one. 
That portion of this paper which deals with the Neocomian, is, I 
regret to say, the least satisfactory. Only stray specimens seem to 
have been collected, and when all are united, we possess nothing 
approaching the grand series that can be brought together for 
examination from Folkestone, Cambridge, or Blackdown. Add to 
this the very different states in which they are found, that many 
are imbedded in hard rock, and are imperfect, or casts, or com- 
pressed, and it will be seen how relatively unfavourable are the 
conditions for determining their limits of variation. By using 
great care, however, and comparing them again and again, I believe 
results are now arrived at which will be upheld by any future 
investigation. 
Of the group of ‘“ Ovate ” we have apparently four distinct Neo- 
comian types. Of these WV. planata seems almost ubiquitous, ran- 
ging from Speeton to Atherfield, whilst the range of the remainder 
seems considerable, though scarcely yet ascertained. They may be 
roughly distinguished from each other as follows :—J. planata is 
the most ovate and compressed ; WV. Cornueliana is thicker and more 
subangular ; lV. simplex has the beak almost overhanging the front 
margin, and is not at all produced anteriorly ; and WV. Meyervis easily 
identified by its tall triangular form and well-defined, heart-shaped lu- 
nule. “A Nucula subtrigona occurs in several British lists; but I have 
not been able to ascertain that it is distinct from the preceding. This 
appears to include all the well-defined species of the continent. The 
group “ Angulate,” it is strange to find, is so far totally unrepre- 
sented in the Cretaceous series of England or France, earlier than 
the Gault. The Lede, four in number, require, with the exception 
of L. spathulata, very careful comparison to distinguish them from 
those of the overlying series, but are easily separable from their 
companion species in the same rocks; the only possibility of con- 
fusion which might arise being perhaps between the young of L. 
spathulatu and L. scapha. It is not improbable that further col- 
lecting may bring new species to light in so varied and extensive a 
series of strata as our Neocomian and Aptian, but the complete list 
of those known to me at present is :— 
N. planata, Desh. L. Seeleyi, Gardn. 
N. Cornueliana, d’Orb. L. subrecurva, Phillips. 
N. simplex, Desh. L, scapha, d’ Orb. 
N. Meyeri, Gardn. L. spathulata, Forbes. 
The Gault forms are in such perfect preservation and so tho- 
roughly well defined from each other that confusion is impossible, 
