I'm not sure whether or not Shiki qualify as a separate species to humans. Total reproductive isolation between two organisms is usually an indication of separation between two species, but this may be that Shiki may simply choose not to interbreed, if it is even possible for them to breed at all. However, Shiki are simply undead humans, so they would still share many of the same genes as humans, even if they do not share a gene pool. It may be more accurate to label them as a separate race rather than species, but "race" implies that Shiki would still be considered human. Perhaps it would be better to classify Shiki as a subspecies? Yet due to the rules of biological classification, a species must either have at least two subspecies or none at all, meaning that Jinrou would have to be classified as separate from normal Shiki, which just raises more questions of similar nature. This is certainly an interesting species complex in any case. This seems like the sort of problem which would need to be solved by an actual biologist, but clarification of any sort is more than welcome.