Never seen that before - TT150

I've been playing with my macro/micro lens for today's Tiny Tuesday challenge and when I was looking at one of my small shells with a hole near the top I thought it might be interesting to make a macro of the inside. I thought the small things inside were grains of sand but they are something else altogether ! I never knew !  Is there any shell collector who knows what this is? If you want to see what the shell looks like from the outside see the extra.
With thanks to InTheHills who is hosting Tiny Tuesday this month and came up with such a fun theme !

Thanks very much for your kind comments and stars for yesterday's gardening mono :-)