During the Pesach vacation, we visited the Biblical Museum of Natural History in Bet Shemesh. It's a really interesting museum, with live animals, and a guided tour that exhibits animals from the time of the Tanach, and explains both their religious and scientific background. I found particularly fascinating the cases where the the biblical word for an animal now refers to something else.
One of the surprises for me was the tzav צב. In Modern Hebrew, that word unquestionably refers to the turtle. But in the museum, we instead found a large lizard. How did this happen?
First of all, let's see why it originally meant a kind of lizard. This is relatively easy to determine, as we have the Arabic cognate dabb, meaning lizard. Kaddari says it refers to a large lizard from the Agamidae family, and the Daat Mikra on Vayikra 11:29 (the only mention of the tzav in the Tanach), more specifically identifies it with the Egyptian dabb lizard (also known as the Egyptian Mastigure), Uromastyx aegyptia, known in Arabic as dhab. It is found in the desert areas of this region, and is eaten by Bedouins (although this is prohibited to Jews in that verse in Vayikra).
Rashi writes that the "only" meaning of tzav is covered, and his decisiveness here may have caused later writers to view the turtle as the likely "covered" reptile. (In most editions of Rashi, he actually identifies the tzav in Vayikra as froit, Old French for frog or toad. However, Moshe Katan in his explanations of the foreign words in Rashi, printed in the introduction to the Daat Mikra, writes that some manuscripts of Rashi have the word tartuge, meaning "turtle".)
Over the years there have been some attempts to use the word shalchufa שלחופה for tortoise (based on the Arabic), but it has never caught on.
So now when you read Parashat Shemini, where it mentions the tzav, you'll have a better idea of what is referring to. But don't worry if you didn't pay attention last week, during Parashat Tzav - that's a different spelling (צו) entirely...