Isurus planus. labial

Could this possibly be Isurus hastalis or have I got it right?

Looks more like an Isurus planus tooth to me. Google Isurus planus tooth and I think you will find it looks more like those. The I. desori is usually much more slender of a profile and I. hastalis is broader and more straight. Even the lateral I. hastalis teeth don't have this much curve. Some refer to the I. planus as the "hooked-tooth mako".

Yeah, that's planus for sure. I've collected lots of those in the Sharktooth Hill Bonebed. The species has been reassigned to Carcharodon because it is apparently a descendant of hastalis but not different enough to warrant a separate genus.

In California you get oxyrinchus (=desori) in Early Miocene beds but no hastalis nor planus. It's the same in the Early Middle Miocene (Olcese Sand, Lower Round Mountain Silt) but then you get fewer oxyrinchus and lots of hastalis and planus in the Middle Miocene Sharktooth Hill Bonebed (Upper Round Mountain Silt)..