||May RM. How Many Species Are There on Earth? Science. 1988 Sep 16 241(4872):1441-1449PubMed ID17790039
||G. C. Ray, Man and the Sea—The Ecological Challenge, Am. Zool. 25, 451 (1985). doi: 10.1093/icb/25.2.451
||Ray (primary ref) has observed that although the sea contains only 20% of all animal species, it contains systematically higher proportions of higher taxonomic units, culminating in 90% or more of all classes or phyla (largely because all phyla are found in the sea, and the bulk of classes are exclusively marine).