>After all these years I still have trouble knowing which valve of a bivalve
>is the right, which is the left, which side is anterior, and which is
>posterior.  So, I'm trying to document this for my and others' use.  I've
>posted a start on my website.  I'm requesting those of you who know bivalves
>to review my work, offer corrections, and make contributions and/or
>comments.
>
>http://z14.invisionfree.com/Conchologist_Forum/index.php?showtopic=546
>
>Marlo
>merritt island, fl
>Visit my website: www.letstalkseashells.com



In in the vast majority of bivalve forms the umbones (beaks) are
coiled in such a way that they are inclined anteriorwards; that is,
they are prosogyrous. Veneridae is a good example; Mytilidae and
especially Pinnidae are extreme examples. The umbones are also
generally siuated closer to the anterior end than the posterior.

In some forms the umbones are not inclined either anteriorwards or
posteriorwards, resulting in an equilateral shell eg Pectinidae, many
Glycymerididae.

In a very few the valves are posteriorly-inclined or opisthogyrous;
eg Thracia, Myadora, Cuspidaria.  In these forms the umbones are
still generally nearer the anterior end than the posterior.

Forms with inclined coiling have inequilateral valves ie the front
half is not an approximation of a mirror-image of the rear half.

In many forms eg Lucinidae, Veneridae there is a small area called
the lunule just in front of the umbones; this is usually defined by a
thin spiral groove and when the valves are articulated this area is
seen to be cordiform (heartshaped).

Some forms have unequally-sized valves, and are called inequivalve.
Examples are Cucullaea, Corbulidae, Myochamidae. The difference
between left & right valves may be that one is slightly smaller than
the other, and fits within the larger valve, as in the first two
mentioned. Or it may be a difference in inflation; in Myochamidae
Myadora has a flat left valve and gently inflated right, Hunkydora
has a less-inflated left valve and Myochama is cemented by its right.
Oysters (Ostreidae, Gryphaeidae) often have a deeper attached valve
and flatter unattached "lid". Pecten has a flat or even concave left
valve and inflated right. Chama and the unrelated Cleidothaerus are
strongly inequivalve. Many tellinids have a posterior end twisted
slightly to the right.

Some forms  have a resilium (which looks like a little wedge of black
rubber between the teeth of the hinge; not visible from the hingeline
with valves articulated), which functions to open the valves when the
adductor muscles relax. The resilium is attached to a socket in each
valve, the resilifers. In Pectinidae the resilium is triangular and
radially-oriented. In forms such as Mactridae the resilifer is
elongated and the lower (larger) end is inclined posteriorward.
Veneridae lack resilifers.

When I position a bivalve say for lateral photography I try to keep a
standard orientation. The longitudinal axis of the valves is
irrelevant, as it is not normal to (90º to) the vertical axis in
forms with anteriorward-located umbones eg Pinnidae. Therefore I
orient using the hingeline; I place this as close to horizontally as
possible, with the umbones upward. This gives the standard position
for venerids, pectinids etc. I usually see Mytilidae, Pinnidae, true
Cardita and other extreme forms positioned with the longer axis
vertical; this is incorrect, and the hinge should again be
horizontal, with the umbones to the right or left. Limidae are
difficult... it can be hard to tell which end is anterior and which
posterior; if there is a byssal gape this will be anterior (in almost
all forms, not only Limidae but excluding Arcidae), but not all
limids have one. The longer flatter "side" is anterior in limids;
correctly oriented, a limid appears to sit at a steep angle, leaning
back languidly. Fortunately limids have a well-developed hingeplate,
the lower edge of which should sit horizontally.

Positioning a pair of spread articulated ("butterfly") valves,
standard orientation is anterior ends down, hinge vertical, so that
the valves are left on right side, right valve on left.
--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
Seashell, Macintosh, VW/Toyota van nut
‚ Opinions stated are mine, not of the University of Otago
"There is water at the bottom of the ocean"  - Talking Heads

----------------------------------------------------------------------
[log in to unmask] - a forum for informal discussions on molluscs
To leave this list, click on the following web link:
http://listserv.uga.edu/cgi-bin/wa?SUBED1=conch-l&A=1
Type your email address and name in the appropriate box and
click leave the list.
----------------------------------------------------------------------