I have often been puzzled by the Medieval and ancient Hellenistic use of the terms Genus and Species, as though these are synonymous, and not quite synonymous. I found a passage in Diogenes Laetrius which explained the difference succinctly.
A species is that which is included by a genus, as man is included in animal. The most generic is that which, being a genus, does not have a genus, i.e., being; the most specific is that which, being a species, does not have a species, for example, Socrates.
~Diogenes Laetrius, Logic and the Theory of Knowledge, 7.61
Thus, a species is that which is subsumed into a genus, and a genus is that which possesses species. For instance, cosmological arguments are species of the genus of arguments in Natural Theology, whereas kalam cosmological arguments are species of the genus of cosmological arguments. In grammar as well, the passive predicates such as “am seen” is a species of the genus of predicates, as are the active predicates such as “seeing”. Being is itself a genus to the exclusion of a species, as an individual angel is a species to the exclusion of being a genus.
Being thus seems to be the supreme genus in Ontology, while perhaps the supreme genus in Metaphysics is matter (since matter is the principle of individuation, and while ontology is the study of being, metaphysics is the study of that which exists).