Translated by Bl. Jackson.
80 Pages (Homilies: 1, 2, 3, 4, 5, 6, 7, 8, 9)
2. "Let the waters bring forth moving creatures after their kind." God caused to be born the firstlings of each species to serve as seeds for nature. Their multitudinous numbers are kept up in subsequent succession, when it is necessary for them to grow and multiply. Of another kind is the species of testacea, as muscles, scallops, sea snails, conches, and the infinite variety of oysters. Another kind is that of the crustacea, as crabs and lobsters; another of fish without shells, with soft and tender flesh, like polypi and cuttle fish. And amidst these last what an innumerable variety! There are weevers, lampreys and eels, produced in the mud of rivers and ponds, which more resemble venomous reptiles than fish in their nature. Of another kind is the species of the ovipara; of another, that of the vivipara. Among the latter are sword-fish, cod, in one word, all cartilaginous fish, and even the greater part of the cetacea, as dolphins, seals, which, it is said, if they see their little ones, still quite young, frightened, take them back into their belly to protect them. 
Let the waters bring forth after their kind. The species of the cetacean is one; another is that of small fish. What infinite variety in the different kinds! All have their own names, different food, different form, shape, and quality of flesh. All present infinite variety, and are divided into innumerable classes. Is there a tunny fisher who can enumerate to us the different varieties of that fish? And yet they tell us that at the sight of great swarms of fish they can almost tell the number of the individual ones which compose it. What man is there of all that have spent their long lives by coasts and shores, who can inform us with exactness of the history of all fish?
Some are known to the fishermen of the Indian ocean, others to the toilers of the Egyptian gulf, others to the islanders, others to the men of Mauretania.  Great and small were all alike created by this first command, by this ineffable power. What a difference in their food! What a variety in the manner in which each species reproduces itself! Most fish do not hatch eggs like birds; they do not build nests; they do not feed their young with toil; it is the water which receives and vivifies the egg dropped into it. With them the reproduction of each species is invariable, and natures are not mixed. There are none of those unions which, on the earth, produce mules and certain birds contrary to the nature of their species. With fish there is no variety which, like the ox and the sheep, is armed with a half-equipment of teeth, none which ruminates except, according to certain writers, the scar.  All have serried and very sharp teeth, for fear their food should escape them if they masticate it for too long a time. In fact, if it were not crushed and swallowed as soon as divided, it would be carried away by the water.
 Narrated by AElian (Anim i. 16) of the "glaucus," a fish apparently unknown.
 Maurousioi. cf. Strabo, ii. 33.
 e.g. Arist., De Anim. viii. 2 and AElian, ii. 54.
Reference address : https://www.elpenor.org/basil/hexaemeron.asp?pg=60