Whatsoever hath no fins nor scales in the waters,.... Which is repeated that they might take particular notice of this law, and be careful to observe it, this being the only sign given:
that shall be an abomination unto you; the Targum of Jonathan says, that not only the flesh of such fish, but the broth, and pickles made of them, were to be an abomination; which contradicts what Pliny (t) relates, that the Jews made a pickle of fishes that lacked scales; so Grotius understands him: this law of the Jews is taken notice of by Porphyry (u), who says, it is forbidden all the Jews to eat horse flesh, or fishes that lack scales, or any animal that has but one hoof: and Pliny (w), from an ancient author, Cassius Hemina, makes mention of a law of Numa, forbidding the use of fish that had not scales, in feasts made for the gods.
(t) Nat. Hist. l. 31. c. 8. (u) De Abstinentia, l. 4. c. 14. (w) Nat. Hist. l. 32. c. 2.

Whatsoever hath no fins nor scales, &c.--Under this classification frogs, eels, shellfish of all descriptions, were included as unclean; "many of the latter (shellfish) enjoy a reputation they do not deserve, and have, when plentifully partaken of, produced effects which have led to a suspicion of their containing something of a poisonous nature."