128 J. S. GARDNER ON BRITISH CRETACEOUS NUCULIDZ. 
is a result, however, which we may expect to find of very frequent 
occurrence when species are critically compared. The specimens 
showing this passage are in D’Orbigny’s collection in the Jardin des 
Pianios, 
Tyrer oF N. UvpREsSA. 
Gault Species. 
Nvovra ALsensis, d’Orb., Pal. Frang., Terr. Crét. vol. iii. p. 172, 
pl. 301, figs. 15-17 (1843). Plate III. figs. 6-8, Plate IV. 
figs. 24, 25. 
The average dimensions of this species are tabulated; but I 
possess @ unique specimen reaching a length of 21 mm., a height 
of 16 mm., and a breadth of 10 mm. It is very variable in form, 
more angular and more compressed than JV. ovata, with a very in- 
conspicuous lunule, a glossy and striated surface, and if furrowed, by 
no means deeply so. The test is moderately thick, and the nacreous 
layer very thin. The posterior region seems more developed 
relatively than the anterior, giving the shell an unsymmetrical 
appearance. A variety is more trigonal and tumid, but preserves 
the ill-defined lunule (Pilate ITT. fie. 7); an excephiontl specimen 
modifying the relative measurements in a remarkable degree, 14 mm. 
in lensth, 12 mm. in height, and 8 mm. in breadth, with a less glossy 
shell. The species appears to be very abundant at Cambridge, for 70 
specimens were sent to me for examination by Mr. Jesson. These 
casts, Plate IV. figs. 24-25, average rather smaller dimensions, agree- 
ing better with those given by D’Orbigny, and are easily distinguish- 
able from casts of IV. bevirgata (Plate IV. figs. 20-22), by the much 
slighter prominence of the adductor muscles, and the more com- 
pressed form of the valves. It is one of the most abundant fossils 
at Folkestone, though possessing, according to Mr. Price, only a 
limited range towards the upper part of the Lower Gault, and is 
met with in the same condition as WV. ovata. Of three specimens 
from the Grey Chalk, one appears identical with the present species, 
while the others approach more closely perhaps to WV. «mpressa from 
Blackdown. A specimen from Speeton, in the British Museum, 
labelled NV. subangula, MS., resembles this form, but may be an 
imperfect specimen of JV. planata. 
Blackdown Species. 
Nucura mmpressa. Sby., Min. Conch. vol. v. p. 118, pl. 475, fig. 3 
(1823). Plate IV. figs. 9-12. 
This is a very variable shell, greatly resembling NV. albensis, but 
is smaller, broader, more beaked anteriorly, and possesses a much 
more sunken and strongly defined lunule. The lines of growth are 
more pronounced, and the valve is divided into two regions by a 
deep concentric furrow situated nearly midway between the umbo 
and the inferior margin, the upper region being smooth, and the 
lower traversed by other more or less pronounced furrows in the 
direction of the first and repeating the contour of the shell. 
