J. 8. GARDNER ON BRITISH CRETACEOUS NUCULIDZA. 33 
lunule; the umbones frequently abraded ; interior smooth, nacreous ; 
margin denticulated. The width and prominence of the ribs is 
about the same as in JV. pectinata, but their arrangement differs 
essentially. The anterior and posterior regions are ribbed at 
different angles, so that the two sets of ribs meet and form a series 
of cheyrons or inverted Y¥’s. In all the specimens I have seen the 
umbos are abraded, and it is therefore impossible to see how the orna- 
ment originates. In some cases the ribs diverge or converge twice, 
so as to constitute a region in which they form a series of inverted 
Ws, but the double divergence is seldom extended to near the umbo. 
Pl. IIT. fig. 14 presents a very distinct example of this highly orna- 
mented variety, named NV. ornatisima by D’Orbigny. Sometimes the 
ribs meet for short distances all over the shell, and in one case there 
are five series of diverging ribs side by side. The divergence takes 
place centrally or nearer the posterior extremity; but I have an 
example in which a secondary divergence takes place close to the 
umbones. The sculpture differs in plan, however, even on the two 
valves of the same shell. As the ribs cross the ridge of the lunule 
they bifurcate (Pl. III. fig. 12 a), so that the lunule is twice as finely 
ribbed as the rest of the shell, and the same thing occurs towards 
the posterior margin as far as the hinge extends, and occasionally 
elsewhere. There is this important difference, that while in JV. pecti- 
nata the ribs are parallel with the margins on both sides as far as the 
hinge extends, and are only at right angles to the inferior margin, 
in IV. bwirgata the ribs abut at right angles to the whole periphery 
of the shell. 
The variability of the ornament completely precludes its use for 
the subdivision of the species, and there is hardly an adult shell 
which does not tend to assume the “ ornatissima” character, at 
least along its inferior margins. D’Orbigny considered ‘“* WV. orna- 
tissema” to be a smaller shell than N. bwirgata; but my largest 
specimens are all of the more richly sculptured type, to which about 
ten per cent. of the whole may belong. 
A finely ribbed surface all over the shell seems to have been 
required in the economy of the animal ; for the chevron-like breaks 
prevent the ribs from becoming coarser as the periphery enlarges. 
The interspaces are very finely transversely striated. The cast is 
much smaller, more compressed, and with far less space between 
the inferior margins of the valves than in JV. pectinata; and these 
characters, in addition to those mentioned *, seem to distinguish it. 
It appears to be very common at Cambridge 7 (Pl. LV. figs. 20-22), 
Mr. Thos. Jesson, F.G.S., having sent me no fewer than 260 speci- 
mens, four of which have portions of the test adhering ; and though 
there is considerable difference in the form of the cast, I do not 
think they represent more than one species. It ranges, according 
to Mr. Jukes-Browne, through the Upper and Lower Gault at Cam- 
* See iV. pectinata. 
t It was identified with NV. simplex of the Neocomian by Prof. Seeley; but 
the error was pointed out by Mr. Jukes-Browne, Quart. Journ. Geol. Soe. vol. 
xxxi. p. 299, who figured several specimens (pl. xv. figs. 4-8). 
