Choosing the best shell for extracting muka fiber

In New Zealand, we use (among other native fibres) Harakeke/Phormium Tenax/NZ Flax for our weaving. Some (not all) varieties have a really good natural fibre in their leaves that we can extract and use for fine garments and belts, handles, cloaks and many other things. This Fiber is called muka. To extract this fibre we use traditionally a mussel shell. This is a short introduction on how to choose the right shell for that mahi/work.