As Andrew indicated, "spondylus" can mean either a joint or a vertebra. While the bivalve genus Spondylus does have a very intricate and impressive ball and socket hinge, and that "joint" may have inspired the name, I think the alternate meaning, vertebra, is the more likely source. A vertebra consists of a central disk-like structure, from which processes protrude at various angles. In human vertebrae the processes are rather short and blunt, but in some species, especially fish, they are quite long and slender, and I suspect that this similarity inspired the name.