Update: 2013.09.29

TERMINOLOGY

aculeate - having spines or prickles
acute - sharply angles
adapertural - toward the aparture of the shell
adapical - along the axis, or toward the apex
adaxial - internal toward the shell axis
adductor scar - impression on interior shell where adductor muscles attach
albino - white due to lack of colour pigments
angulate - angled, formed with corners
angustate - narrow, compressed, or diminishing rapidly in breadth
anterior - forward, front or head end (opposite of posterior)
antero- - combining form of anterior (e.g., anterodorsal)
antimarginal - sculpture more or less perpendicular to valve margin
arcuate - curved as a bow, or arched
auricle(s) - ear-like projections on the anterior and posterior ends of the hinge line (also ears)
auricular – pertaining to the auricle
auricular crura - tubercles and ridges inside the auricles
auriculate – having ear-like appendages
axial – refers to longitudinal sculpture, parallel to the lengthwise axis of the shell

bathymetric - pertaining to the measurement of depth of water
beak - projecting region above the hinge marking the earliest growth of the shell (see also umbo)
biangular – having two angles or double keeled
bifid - divided into two parts by a groove
bifurcate - divided into two, especially in sculpture
bipartite – having two distinct parts
biplicate – folded twice
blotch - irregular dot of contrasting colour
bridged - partly covered
bulbous - globular
byssal fasciole - track left by developing byssal notch
byssal gape - opening remaining when the shell is closed for passage of the byssus
byssal notch - gap near anterior auricle on right valve for passage of the byssus
byssal sinus - embayment below anterior auricle on left valve corresponding to the byssal notch of the right valve
byssate - with a byssus
byssus - a bundle of fibers to anchor to the substratum

calcareous - composed mostly of calcium carbonate
calcite - rhombohedral crystalline form of calcium carbonate
cancellate - having a pattern of intersecting radial and concentric ribs (see also reticulate)
cardinal - central part of the hinge area directly below the beaks
cardinal crura - narrow plate-like teeth radiating from the resilifer
cardinal teeth - teeth directly below and radiating from the umbo
carina - a prominent, sharp-edged or keel-like radial ridge on the surface of the shell
carinate - with a carinae
castaneous - sepia or chestnut coloured
chevron - a V-shaped pattern
cementation - fixation to the substratum by shell secretion
cline - a geographically and gradually developing difference in appearance of certain populations of the same species
collar - a raised lip bordering a suture
commarginal - parallel to the ventral shell margin
commensal - living with another animal, a form of symbiosis
compressed - flattened
concave - curved or arched inwards
concentric - nearly commarginal, paralleling the ventral margin
congeneric - belonging to the same genus
conspecific - belonging to the same species
convex - inflated in left-right axis
cord - a rib-like feature, but smaller
corneus - horny in texture
corrugated - folded or ridged, broadly and haevily sculptured
costa (pl. costae) - a rib-like sculptural element
costate - ribbed
costella - smaller costae
costellate - finely ribbed
co-type - term no longer in use for a paratype or syntype
crassus - coarse or thick in sculpture
crenate (or crenulate) - with regular notches or corrugations
crispate - having wrinkled, ruffled or fluted margin
crura (singular, crus) - lamellar ridges on the hinge plate
ctenolium - row of small teeth on edge of byssal notch
cuneate - wedge-shaped

decussate - having a latticed surface formed by the intersection of fine ribs
demarcated - set off, as by a ridge, furrow or abrupt change in sculpture
denticle - low, rounded tooth
denticulate - having little teeth or fine raised points
denticles – tiny teeth or protruding spikes
dentition - referring to the vertical hinge teeth
depressed - low in proportion to diameter
diameter - measure of shell convexity
diffuse - widely spread out or extended
disc (or disk) - central part of the external surface of the valve
dissoconch - post-larval shell
divaricate - antimarginal or diverging sculpture, often forming chevrons
dorsal - toward the hinge line

ears - lateral projections of the hinge line (see also auricles)
echinated - having the surface covered in spines
ecomorph - a morphology resulting from living in a particular place
edentate - without teeths or folds
elongate - extended, considerably longer in one dimension than another
emarginate - having the margin interrupted or notched
epifaunally - living attached to the substratum
equidistant - equally spaced
equilateral - anterior and posterior size of the shell nearly equal in size
equivalve - with valves equal in size and shape
evanesce - to disappear gradually
exiguous - small or diminitive
extant - not extinct

fascia - stripes or bands
fasciated - marked with transverse bands of colour
fascicle - a small bunch or radial ribs
fente - opening or slit which appears on the slopes of bivalves near the hinge when valves are articulated
ferrigunous - rust coloured
flabelliform - radiating structure from single point or fan-shaped
flexuous - with a bend or curve in the shell
fold - a broad undulation
foliated layer - shell microstructure of lath-like calcitic crystals that form sheets like roof shingles
fossa - shallow depression or cavity
fossula - small pit with reduced partitions
foveolate - having small pits or depressions

gape - opening left between valves when shell is closed
gibbous - swollen unevenly
glabrous - hairless or smooth surface; also pale in colour
globose - roughly spherical shape
granosus - covered with tiny grain or bead-like elevations
granular - bearing granules as surface sculpture
granulate - grainy texture
granule - a pustular surface sculpture
growth striae - commarginal lines on the shell surface that mark growth intervals

hastate - spear-shaped or triangular
height - the greatest dorsoventral dimension perpendicular to hinge line
heterodont - with distinct, differentiated cardinal and lateral teeth
hinge - collective term for structures of the dorsal region that function in opening and closing the valves
hinge line - antero-posterior axis of the hinge
hinge teeth - shelly structures fitting into sockets in the opposite valve and serving to assure accurate closure
hyaline - thin and nearly transparent
hypertrophied scales - inflated

imbricated - overlapping or shingle-like
immaculate - without a blemish or mottling of colour
immarginate - having no elevated or coloured margin
impressed - marked by a furrow, pressed below the surface
incremental lines - faint concentric growth lines
inequilateral - anterior and posterior sections of the valves dissimilar in shape and size
inequivalve - having the two valves dissimilar in shape and size
inflated - swollen or strongly convex
intercalary - between (e.g., smaller ribs between two major ribs)
intercostal - between ribs
intercostate - between ribs or ridges
interspaces - region between adjacent ribs
interstices - interspaces, but narrower and deeper
interstitial - channels between ribs

lamella (pl. lamellae) - thin, raised ridge(s) of external sculpture
lamellate - covered with scales
lamellose - with thin scales or plates
lamina (pl. laminae) - thin, raised ridge(s) of the hinge
left valve - upper valve
length - dimension along antero-posterior axis
lenticular - flattened and generally circular
ligament - elastic proteinaceous structure joining the two valves dorsally
ligamental pit - broad subumbonal depression accommodating a ligament
lira (pl. lirae) - a threadlike sculptural element
lirate - having a threadlike sculpture
littoral - bathymetric range between tides and subtidal zones
longitudinal - direction of ribs or lines from hinge to ventral margin

macula - large colour blotch
maculated - splashed or blotched with colour
maculations - spots, generally of irregular size, shape and distribution
maculose - covered with spots or blotches
margin - the edge, particularly of a shell surface
medial - central
miniaceous - red lead colour
monophyletic - with a single evolutionary origin
morphological - pertaining to structure or form
multistriate - having many thread-like lines, grooves or scratches
muscle scar - impression on the interior of a shell marking the former place of attachment of muscle
muricate - having sharp elevated points

nacreous - shell microstructure of thin aragonite layers, characterized by iridescence
nestling - fixed in a crevice or crack
node - a small knob
nodose - sculpture of nodes or tubercles
nodulous - having nodules or knobs

oblique - intermediate between dorso-ventral and antero-posterior direction
obsolete - sculpture that disappear or remain undeveloped
obtuse - blunt or rounded at the extremity
ocelli - minute, simple eyes
ochraceous - pale yellow to brownish yellow
ontogeny - history of evolution of an individual organism
opisthogyrate form - with beaks pointed backwards, or posteriorly
orbicular - circular in outline
ovate or ovoid - having an oval shape

pallial line - scar on valve interior from attachment of marginal pallial muscles
pecten - comb-like, from Latin
pelagic - living in the open ocean
pelecypoda - a bivalve, which is hinged along one edge
pellucid - transparent or translucent
phylogeny - history of the ancentral development and relationships of a species
plica - a fold or corrugation
plicae - folds
plicate - folded or twisted lengthwise
polymorphic - having many forms
polyphyletic - with more than one unrelated origin
ponderous - heavy or bulky
posterior - towards the back end (opposite of anterior)
postero- - combining form of posterior
prismatic - shell microstructure consisting of calcitic or aragonitic prisms
prodissoconch - the shell secreted by the larva, preserved at the tip of the umbones
profuse - abundant or many
punctate - surface with minute pits
pustule - knobby sculpture, generally smaller than a tubercle

radial - radiating in relatively unbroken lines from the umbones to the ventral margin
resilifer - a structure that supports the internal ligament
resilium (pl. resilia) - internal portion of the ligament
reticulate - external sculpture of a network of intersecting elements
rib - an elongated radial or concentric structural element
riblet - a narrow or rudimentary rib
right valve - lower valve
rosaceous - deep rose-pink coloration
rostrate - pointed or beaked
rotundate - having a rounded shape or being nearly circular
rugose - having parallel ridges or wrinkles on the surface

scabrous - rough, scaly
scale - a localized projection of shell sculpture
sculpture - relief pattern on the shell exterior
serrate - a saw-like margin
sessile - attached to some substrate, not free to move
shagreen - net-like, honeycomb-like or reticulated exterior sculpture
sinus - a recess or indentation
spine - long calcareous projecting sculpture
spinose - having spines or thornlike protuberances
spinulous - having small sharp spines
squamose - sculpture of small scales
stria (pl. striae) - minute linear sculpture, indication a growth stage
striate - marked with a sculpture of fine scratches, grooves, or lines
striga - a narrow transverse line or streak
strigate - marked with fine closely set grooves
sub- - combining form meaning nearly (e.g., suborbicular)
sublittoral - bathymetric range between the littoral and bathyal zones
substratum (pl. substrata) - bottom sediment
symbiotic - two dissimilar organisms living in intimate or close association

tenuis - thin
tenuous - delicate or thin
tessellated - having a checkered pattern
testa - shell or rigid external covering of mollusca 
thread - narrow rib
translucent - semitransparent, allowing some light to pass through
transparent - allowing light to pass through undistorted
transverse - crossed or across
trifid - having three parts or sections
trigonal - triangular in outline
tripartite - divided into three segments
truncate - sharply or squarely cut off
tubercle - a small knob or lump
tuberculose - having small knobs or projections
tumid - abnoemally swollen

umbo (pl. umbones) - the upper part of a valve as seen from the outside
umbonal - situated near or relating to the umbo
undulated - waved

variant - a shell differing slightly in form or colour from other † shells of the same species
variegated - with patches of different colours
ventral margin - the lower margin, opposite to the hinge line
ventricose - strongly inflated or swollen
vitreous - glass-like or transparent

width - the antero-posterior dimension of the shell

xenomorphic - assumption in form and sculpture of the substratum