From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer shares extensive chiddushim on the sugya of simanei beitzim and simanei dagim in Chullin 64 and Avodah Zarah 40, addressing whether these identifying signs have Biblical or Rabbinic force. He supports R' Tam's proof that fish signs are d'oraisa, explains the Tur and Rosh on using fish signs to permit roe, and offers novel explanations regarding the word 'bas haya'anah' and the natural development of chicks in kosher vs. non-kosher birds.
Source: Shu"t Chasam Sofer YD §78