134 J. S. GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 
bridge. It is limited by Mr. Price to the 7th “ bed” of the Lower 
Gault, and the ‘‘ Junction bed” at Folkestone; I have, however, also 
found it there in the basement bed and, I believe, in the Upper Gault. 
H. and A. Adams proposed a subgenus, Acila, to receive the 
present and other species with diverging ribs. 
Biackdown Species. 
Necvra antievaTa, Sby., Min. Conch. vol. v. p. 118, pl. 475, fig. 4 
(1823). Plate IV. figs. 3-8. 
This is another extremely variable species, its height sometimes 
equalling its length. The umbones are very far forward, even 
occasionally overhanging the lunule, which is heart-shaped, as wide 
as high, flattened and sharply defined. The surface of the shell is 
slightly ribbed and strongly reticulated by lines of growth with 
occasional grooves or furrows; but when the outer surface is re- 
moved, a deeply pectinate sculpturing is exposed without any trans- 
verse lines. The lips are denticulated, and the hinge is frequently 
well exposed. It is only known from Biackdown, where it seems to 
be a not uncommon shell. WN. apiculata, Sby., Geol. Trans. 2nd ser. 
vol. iv. p. 342, pl. xvii. fig. 10., is supposed by Mr. Downes to be a 
synonym for this species; but I have not come across the original, 
and no specimen of it exists in any museum to which I have had 
access. The pectinate structure sometimes exposed (PI. IV. fig. 6) 
through decay of the surface doubtless led Dr. Fitton to include 
NV. pectinaia in his list of Blackdown fossils. 
Subdivision ANGULATZ LEVIGATS. 
Gault Species. 
NucvLa GAULTINA, sp. nov. Plate III. figs. 9-11, 26, 27. 
LV. impressa, auctorum, of the Gault, non WV. impressa, Sby., from 
Blackdown. 
This is a small triangular somewhat tumid species, with very 
distinctly separated, heart-shaped lunule; smooth and polished, 
finely striated test, crossed by almost invisible pectinate markings, 
and one or more deep furrows; nacreous internally with finely den- 
ticulate inferior margin. As in J. antiquata, a pectinate structure 
is completely masked by a thick outer shelly covering. The cast 
is smooth, with the scars of the adductor muscles very indistinct, 
and the denticulated margin only visible with a lens. It is a much 
smaller species than Sowerby’s Blackdown JV. umpressa, with which 
it has so frequently been confounded, only one specimen out of a 
large number reaching 10 mm. in length by 8 mm. in height and 
6 mm.in depth. It is also more triangular, more truncated ante- 
riorly, with a less curved and denticulated inferior margin and 
larger and more defined lunule. It.is an exceedingly abundant 
fossil at Folkestone, though limited apparently to the Lower Gault. 
A more elongated form with less-defined lunule is very rare, and 
is considered a distinct species by Dr. Gwyn Jefireys. (Plate LI. 
figs. 26, 27.) 
