On Fri, 26 Jun 2020 23:34:45 -0700 (PDT), polygonum_on_google wrote:

Therefore, I rather assume they are some sort of cast. Possibly
some
sort of protein binding it all together.

Cerainly look constructed rather than just a cast. That plain edge
on
the left of the close up, the patterning over the bulk. Are they
hollow? Could they be the "shells" of something akin to a cadis
fly
larva?

They are approximately one millimetre thick and consist only of what you
can see.

Hum, looking at the overview image, top right there is one that looks
to be less damaged and appears to be more of a hollow cone with a
hole at the top. Bucket and spade...

--
Cheers
Dave.