Dear Paul et al.:
 
Yes, it is a familial character.  I stumbled across Odhner, n. H., 1919
Studies on the morphology, the taxonomy and the relations of the Recent
Chamidae. Kungl. Svenska Vetenskapsakamiens Handligar 59(3): 1-102 + 8
plates and solved my problem.  It is often quite easy ro see the 3.5 mm.
"Irus" on an adult Chamid with use of the stereomicroscope.
 
Harry
 
 
At 09:24 PM 3/17/99 EST, you wrote:
>Harry, I never knew that about Chamidae.  Is this true of all Chamidae, or
>just Arcinella?  I have seen some extremely tiny Chama shells (under 1 cm)
>already attached to larger shells, and looking like typical Chama.  How small
>is the Irus-like stage you mentioned?  And how large are Arcinella when they
>start looking like Arcinella?
>     Andy offered a very plausible theory for the difference in sculpture
>between juvenile and mature burrowing bivalves.  However, the same phenomenon
>is also seen in many non-burrowing gastropods (sculpture on the early whorls,
>with no sculpture or reduced sculpture on the later whorls).  Then again,
>there are gastropods AND bivalves which reverse this pattern, having smooth
>early whorls and sculptured later whorls.  Probably the principle purpose of
>this arrangement is to confuse and frustrate conchologists.  I can't think of
>any other reasonable explanation.
>     One of my favorite examples of "dichotomous metamorphoses", as Harry
>describes the process, is the west coast "rock scallop" Hinnites, which starts
>life as a typical free-swimming Chlamys-like "scallop" shell, but later
>attaches itself permanently to rocks, and grows into a heavy, misshapen
>oyster-like thing.  But of course the all time champions of metamorphosis are
>the bivalves of the family Clavagellidae, the "watering pots", in which the
>juvenile looks like a typical smooth little clam, which however subsequently
>grows into a long, hollow tubelike structure, often with sand grains and other
>miscellaneous objects cemented into it, and a bizarre, frilly "head" at one
>end, perforated with tiny holes.  Now that's metamorphosis!
>Paul M.
>
Harry G. Lee
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