הלכה על חולין 117:30

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Chullin 59a) that fins are what in the fish is like a type of wings with which to fly; and scales are that covering that clings to all of its body. And even if scales are only found on part of its body - and even it is only one - it is enough for it and it is fit (kosher). And they said in Tosefta Chullin 3:9, "And that is when it is under its jaw or under its tail or under its fins; but on the rest of its body, it is not enough for us with less than two." And everything in which scales are found - even one in the places mentioned - need not be checked [to ascertain] if it has fins; as it certainly has [them]. But if fins are found on it, it needs to be checked [to ascertain] if it has scales - as there are many impure fish that have fins. And any fish the nature of which is to have scales, even though they are not produced on it when it is still small until it grows up - such as the sultanit and the afian - behold it is permissible. And anything that has scales when it is still in the sea, even though it drops them at the time that it goes out from the sea - such as the akonas, the afonas, the bisifityas, the afnasetiyas and the tunny that is called biret in the vernacular - behold, it is fit (Chullin 66). As the Torah is only concerned with the species that it have scales and fins - since that is what is seemly for the nature of people, and from the reason that we wrote in the prohibition of foods according to the simple understanding.

From the laws of the commandment is that which they, may their memory be blessed, said (Chullin 59a), "The signs of the the beast and animal are stated from the Torah" - meaning to say they are explicit in the Torah, and that is a hoof completely divided, like the ox and the sheep, and [that] it brings up the cud - "but the signs of the fowl were not stated," since the Torah delineated for us by name all of the impure ones in the world. And since it is so, there is no need to write their signs - as any bird that is found in the world besides them is fit (kosher). And nonetheless since not every man is an expert in all of the species of which the Torah informed us, the Sages informed us of signs through which any man may recognize them and he be able to eat of fowl, even though he not recognize even one of the many impure species that are delineated in Scripture.