Quotation for Shabbat 215:4

איכא דאמרי א"ר זירא אף אנן נמי תנינא בכנפיו לרבות את העור אי אמרת בשלמא עור הוא היינו דאיצטריך קרא לרבוייה אלא אי אמרת לאו עור הוא אמאי איצטריך קרא לרבוייה אמר ליה אביי לעולם אימא לך לאו עור הוא ואיצטריך סד"א כיון דאית ביה פירצי פירצי מאיס קמ"ל

Others state, R. Zera said: We too learnt thus: 'By the wings thereof'; — this is to include the skin. Now, if you say that it is [a separate] skin, it is well: hence a verse is required for including it. But if you say that it is not skin, why is a verse required for including it? Said Abaye to him, in truth I may tell you that it is not [a separate] skin, yet it is necessary. I might argue, Since it is covered with splits [holes], it is repulsive. [Hence] we are informed [otherwise]. Mar son of Rabina asked R. Nahman b. Isaac: May tefillin be written upon the skin of a clean fish? If Elijah will come and declare, he replied. What does 'if Elijah will come and declare' mean. Shall we say, whether it has a [separate] skin or not, — but we see that it has a skin? Moreover we learnt: The bones of a fish and its skin afford protection in the tent wherein is a corpse!<span class="x" onmousemove="('comment',' If food is in a vessel which is covered by the bones or the skin of a fish, or if the whole vessel, which is closed, is made from these materials, the food is protected from contamination; v. Num. XIX, 15. — Thus the skin is mentioned as a separate entity. ');">⁹</span> Rather [he meant]: If Elijah comes and tells [us] whether its foul smell<span class="x" onmousemove="('comment',' Lit., 'filth'. ');">¹⁰</span>

Mishnah Kelim