TT #115 · Parshas Shemini · Aliyah 6 · Passuk 9
English: These you may eat among all that live in water: anything in the water, whether in the seas or in the streams, that has fins and scales—these you may eat.
Chullin 59a.
E-Daf.com — Chullin 59a (Tsurras Hadaf)
סְנַפִּיר וְקַשְׂקֶשֶׂת. מַהוּ סְנַפִּיר וּמַהוּ קַשְׂקֶשֶׂת? סְנַפִּיר הַפּוֹרֵחַ בָּהֶן, קַשְׂקֶשֶׂת הַקְּבוּעָה בּוֹ.
Definition — “Fin” is the swimming appendage, while “scale” is the fixed covering attached to the creature.
The two fish signs are defined as the fin that propels the fish and the fixed scales.