re OCTOCERA. 
deposit of shelly matter to which the successive layers are added ; 
the apex of the spiral cone, of which most shells are composed 
(see Conz.) The nucleus is formed within the egg, in ovipa- 
rous; and within the old shell, in viviparous mollusca. It is 
frequently more transparent and light than the remainder of the 
shell, and sometimes falls off; when this occurs, the shell is said 
to be decollated. 
NU'CULA. Lam. (A small nut.) Fam. Arcacea, Bl. and Lam. 
Descr. Equivalve, inequilateral, transverse, covered with an 
epidermis; hinge linear, with a series of sharp, angulated, teeth, 
arranged ina line on each side of the umbones and a central, 
ligamentary pit; muscular impressions two, simple; palleal 
impression not sinuated. Obs. The row of teeth on each side of 
the umbones, and the ligamentary pit in the centre of the hinge, 
prevent the pretty little shells of this genus from being con- 
founded with any other. Fig. 137, N. fluviatilis. 
NUMMULA’'CEA. Bl. The third family of Cellulacea, Bl. con- 
sisting of the following genera, Nummulites, Siderolites, Vor- 
ticialis, Helicites, Orbiculina, Placentula. 
NUM™MULITES, Lam. (Nummus, money.) Fam. Nautilacea. 
Lam. Deser. Orbicular, convolute, shewing no trace of spire 
externally; interior divided into cells spirally arranged. Obs. 
The singular fossils composing this genus receive their name 
from their external resemblance to a battered coin. Fig. 472, 
N. lenticulina. 
NYMPHA’CEA. Lam. A family of the order Conchifera Dimy- 
aria, Lam. containing the genera Sanguinolaria, Psammobia, 
Psammotza, Tellina, Tellinides, Corbis, Lucina, Donax, Capsa, 
Crassina. To which may perhaps be added Grateloupia and 
Egeria. 
OBLI'QUE. (Obliquus, lat.) In a slanting direction. The whorls 
. of spiral univalves generally take an oblique direction, in refer- 
ence to the imaginary axis of the shell. A bivalve is said to be 
oblique when it slants off from the umbones. 
OBSOLETE. (Obsoletus, lat.) Worn out, out of use. This term 
is used to express acertain indistinctness of particular characters, 
which sometimes results from the action of sea-water upon un- 
protected parts of the shell, and sometimes from the deposits of 
enamel formed in age, and covering the early strive, ribs, teeth 
&c. thereby rendering them less acute. 
OCEA’NUS. Montf. (‘ Corne d’ammon vivant,” Fr.) Naurius 
Umbilicatus, Auct. 
OCTOCE'RA. Bl. The first family of the order Cryptodibran- 
chiata, Bl. containing the genus Octopus. A species of which 
being found in the Argonauta, or Paper Sailor, has given rise to 
the long continued controversy as to whether it is really the 
constructor of the shell, or whether itis a mere pirate, and hay- 
