One of the examples that appears in the Mishna on yesterday’s daf of an abortive birth that is considered significant for the laws of tum’at yoledet – ritual uncleanliness following birth – is “a sandal or a placenta.” The Gemara on today’s daf discusses the appearance of a “sandal.”
Our Rabbis taught: A sandal is like a sea-fish of the same name. At first it is a normal fetus but later it is crushed. Rabban Shimon ben Gamliel said: A sandal resembles the tongue of a big ox.
In the name of our Masters it was testified: A sandal must have facial features. Rav Yehuda cites Shmuel as teaching: The halakhah is that a sandal must have facial features. Rav Adda citing Rav Yosef who had it from Rabbi Yitzhak ruled: A sandal must have facial features even if only at the back, this being a case similar to that of a man who slapped his fellow and caused his face to turn backwards.
The “sea-fish” described as a sandal appear to be a flatfish (bothus) that is flat and has both eyes on one side of its body. (In Hebrew it is called dag Moshe Rabbeinu – “Moses’ fish” – since it appears to have been split in half with the splitting of the Red Sea.)
The disagreement about the definition of “sandal” as a fish or the tongue of an ox appears to relate to both its name and to its appearance. The Sages identify it with a sandal-fish (described above) while Rabban Shimon ben Gamliel suggests that it looks like sandals worn on one’s feet, i.e., like the tongue of an ox. The Arukh LaNer notes that the Rambam (Mishneh Torah, Hilkhot Isurei Bi’ah 10:12) identifies the sandal as having the appearance of the tongue of an ox. Given that there is no compelling reason to follow the opinion of Rabban Shimon ben Gamliel against the Sages, he explains that the Rambam apparently believes that there is no practical difference of opinion regarding the shape of the miscarriage, and the only argument related to the source of the name. The Rambam chose the example that would be more widely recognized.