Marine biologists and hobbyists, I think this is one for you.

My son, who is a serious amateur photographer, took several images of
some organic entities he saw all over several Northern California
beaches (Gualala, Half Moon Bay) in the spring, both April and June. 
He said there were huge numbers of these everywhere, both in a
seemingly fully hydrated state and as completely desiccated paperlike
ghosts, both forms being seen on the same beach on the same day.  They
seem to have a sort of thick gelatinous character and are disclike in
shape, roughly 3" in diameter, with a curious ridgelike vertical
growth a bit like pictures I've seen of Portuguese man-of-war.  Views
from several angles are here:
http://membres.lycos.fr/dbky/Apteryx/SquishyBeachThings
These are full size, so you may want to download them.  If a
researcher needs me to reduce and repost, I will.

We both want to know what they are.  I told my son that some G****e
researcher would surely be able to tell us.

I have not been with him on these excursions, but I have visited other
Northern California beaches at various times of the year.  I have
never seen anything like these anywhere, ever in my life.  As I kid on
the East Coast, I used to see a lot of jellyfish, and there seems to
be a distant similarity here, but these are not like any jellyfish I
know.

Thank you,
Apteryx

Greetings Apteryx,

What a fascinating creature, I myself have never seen one, but I have
identified the little critter for you.

I used the following search terms first: beach animals washed jelly

This let me to the following site: 
http://lanecc.edu/science/zonation/sandybea.htm by the Lane Community
College of Eugene, Oregon.  I viewed several organisms on this page,
and one of the called "Sailor by the Moon" or in Latin "Velella
velella" appeared to be VERY similar to what your pictures show. 
Also, it says on their website "They can be found as far north as
Oregon from the tropical Pacific and on the Atlantic side from the
tropics to Cape Hatteras." The article on this site is very
interesting and also describes their appearance in exacting details,
how they find their way to the beaches, which interesting enough has
to do with the angle of their sail fin; and, describes after their
death on the beach that "...only their cellophane skeletons are left."
 It also describes them as a "purplish-blue elliptical hydroid with a
cellophane-like gas-filled float and triangular cellophane-like sail."

To me, the above description sounded exactly like what you had
photographed and described.  To be sure, I searched google using the
latin name of the organism in question.  Here is the actual search:
://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=Velella+velellas

Here are a few other sites that describe and include photographs of
the organism you have photographed, which is obviously Velella
velellas. It is sometimes also referred to as "Sailor by the Moon, By
the Wind Sailors, & Purple Sailors".

http://www.ci.santa-cruz.ca.us/pr/wharf/critters/critters5.html#bythewind
http://www.ptreyeslight.com/stories/apr11_02/jellyfish.html  (This
site asserts they are relatives of the Portugeuse Man-O-War
jellyfish!)
http://home.comcast.net/~4miller/stories/beachwalk/beachwalk.html

I hope you have found my answer helpful, I'll be glad to clarify in
any way necessary.

Kindest regards,

mosquitohawk-ga

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Request for Answer Clarification by apteryx-ga on 16 Feb 2004 17:02 PST

Sounds good so far, mosquitohawk!  What about the dry ones--what
happens to them, and how come they turn bone white and papery?  That
is a very weird end for something that looks much more substantial
than tissue.

Thanks,
Apteryx

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Request for Answer Clarification by apteryx-ga on 16 Feb 2004 20:38 PST

Wait, never mind.  Between your answer and what czh added, I think
we've got it.  Thanks very much.

---

Clarification of Answer by mosquitohawk-ga on 17 Feb 2004 14:40 PST

From the first major paragraph of my answer:

"describes after their death on the beach that "...only their
cellophane skeletons are left."

;)  Glad you were happy with my answer, thanks for the stars!

Entirely satisfactory.  Accurate ID and full information.  Thank you.

Apteryx

Hello apteryx-ga,

I?ve occasionally seen these intriguing jellies on Northern California
beaches. The drifts of their dried out husks make me think of tiny
ghosts huddling on the sand or hiding amongst the driftwood. Since
most jellies are up to 95 percent water, I think there?s only the
cellophane-like shell left when they?re dried out. Your son?s pictures
captured them wonderfully.

Since they tend to show up in the spring, you might be able to catch
them later this year. Good luck.

~ czh ~


http://www.fitzgeraldreserve.org/seasons.html
Seasons at Fitzgerald Marine Reserve 

Those piles of blue ?jellyfish?
Velella generally remain in offshore waters, however shifts in wind
direction or currents can drive these animals onshore. When the wind
pattern shifts to a northwesterly direction in early spring, and
depending on its strength and persistence and the abundance of Velella
offshore, windrows (narrow rows of deposited material that accumulates
along the upper edge of the previous high tide) of beached by-the-wind
sailors several inches deep can pile up on exposed sandy beaches and
rocky shorelines. These animals create quite a stink for a short time,
but quickly decompose, leaving behind only the thin, light blue,
cellulose-like chitinous exoskeletons which can persist on the beach
for months. These interesting animals are another harbinger of spring
along the Pacific coast.

http://morro-bay.com/docents/curt-beebe/velella/index.htm
Velella Velella
This site offers some wonderful close-up pictures.

Thank you for your addenda, czh.  Together with mosquitohawk's finds,
they give me a really complete picture of these curious creatures.  I
especially liked the image that you can rotate!  I'm satisfied that we
have correct and comprehensive information now to satisfy our
curiosity, and I thank you both.

Apteryx

Greetings Apteryx ...

Your son took some great piccies.

Fascinating!

Bryan