You state that Rashi defines the Chomet as "limace".
The authoritive French Larousse dictionary defines "limace" as a snail without a shell ("sans coquille"), Rashi explicitly states that the Chomet has a shell. The proper French term for a snail with a shell is "limacon" (the "c" with a cedilla). I have to admit that Rashi always includes the "n" at the end of the words, such as in Shabat 54b DH Kupar, regarding the "harisson". More research might reveal that the modern "limacon" was "limace" in Medieval French.
Thanks,
Mordechai Schwimmer, Brooklyn, NY USA
1) J. Greenberg (Foreign Words in Rashi, on Tanach, Yerushalayim 5740) provides a list of the different versions of transliterated Lazim of Rashi for "Chomet" (Vayikra 11:30) and "Shablul" (Tehilim 58:9): limace, limase, limaz(e), lymaz and limazon and limezon. Greenberg chose to translate Chomet as "slug" and Shablul as "snail."
2) Rashi brings limace for both Shablul (in Shabbos 77b) and Chomet (in Chagigah 11a and Chulin 122a) and Chilazon (Avodah Zarah 28b, according to the manuscript version of the Laz, which the translators seem to deem correct), but brings limacon for Shablul elsewhere (Moed Katan 6b). Bereishis Raba 51:1 calls a Shablul "Limaze." (This seems to be a translation by a later commentary which became incorporated into the text of the Midrash.)
Moshe Katan (Otzar ha'Lazim on Shas, Yerushalayim 5748, entry on Chagigah 11a) states that the word seems to mean "slug," as you said, but points out that in Chagigah Rashi clearly refers to a snail.
Dayan Yisroel Gukovitzki (Targum ha'Laz on Shas and Tanach, London 5752) feels, however, that the word in Rashi in all of these places might be mistyped, and it should actually read limacon (the "c" with a cedilla), which he found in one printing of Rashi on Shabbos and in one manuscript of Rashi in Chulin. (He hints that the Girsa in Rashi in Vayikra should also be limacon.)
3) Indeed, it seems quite clear from Rashi that the Chomet and Shablul have a shell. Rashi clearly refers to the shell of the Shablul and Chomet in his comments to the above sources in Moed Katan, Chagigah, Avodah Zarah and Chulin and in Devarim 8:4. So does the Radak in Tehilim (who also refers to it as limace). In contrast, Rashi makes no clear reference to a shell-less slug anywhere in his commentary.
In short, it seems that either the Old French word limace meant both slug and snail, or else our record of Rashi's translations is inaccurate, and wherever our text reads "limace" Rashi actually wrote "limacon." (I found a Sefer called "Lekach Tov" (Harav Chananel Tuvya Dayan, Bnei Brak 5749) that discusses this discrepancy at length at the end of his commentary on Chagigah.)
Y. Tavin and M. Kornnfeld