I'm not doubting you Greg - but I don't remember a mussell shell being shown specifically in the context of a fire lighting kit.
That being said i've given it a bit of thought and all I can come up with are the following thoughts . I don't have an answer (I feel like i'm saying this alot tonight
) but...
In the context of using a mussell shell to catch an ember I can't help but think that it would be a poor tool to catch a spark given the inherently conical shape of the base. If there's one thing i've learn in fire from flint or fire from friction it's that a stable spark platform is essential (Alright you can get away with this with charcloth, but I can't see an application where a mussell shell would be of any benefit as opposed to say putting charcloth on a flint to catch a spark direct)
A Mussell shell certainly isn't made of a material that could be used to produce a spark itself, and as far as I'm aware they are, being basically just calcium, inert as far as fuel-providing properties are concerned.
We have seen a variety of ember carriers in use in bushcraft. I suppose a mussell shell (preferably both halves) could serve in this capacity, but it's insulative properties are not high and it's size would prohibit carrying much of practicable time-value beyond something like King Alfred's cakes. Some of the flat fungi would probably work far better and last far longer on their own with a withy threaded through - airflow in a mussell shell might be too low to sustain even with KA Cakes.
So to sum up. No I can';t answer your question. It has made me think about some things though. Which is what all good questions should do. So cheers for that.
Maybe Rich59 will have some thoughts. He's our resident firestarter after all. :notworthy