The shell on the left belongs to the freshwater snail Lymnaea stagnalis, while the one on the right is that of Helix aspersa, a land snail. And here is what you would get if you could put the two together.
I found this specimen in the land snail collection of the Smithsonian's Museum of Natural History. Tim Pearce and I examined it briefly. We came up with 3 explanations as to how it may have been created.
1. It is a fake. If it is a fake, it is a very cleverly made one. As you can see in the picture below, the body whorl of Helix grew over the body whorl of Lymnaea. To be able to do that one would have to remove the body whorl of a Helix shell by cutting along the suture of the body whorl, put the rest of the shell inside a larger Lymnaea shell, place the body whorl of Helix on the outside and, finally, glue everything together without leaving any traces of the cuts and breaks.
2. Tim suggested that a juvenile H. aspersa in its shell was purposely lodged, or glued, inside an empty L. stagnalis shell. The Helix couldn't get out, so it grew out of the Lymnaea shell.
3. I thought that a H. aspersa was removed from its shell and placed inside an empty L. stagnalis shell. Somehow, it survived, reattached its columellar muscle and continued to grow, building its own shell. In Tucker Abbott's Compendium of Landshells (p. 193) there is a picture a similar chimera of H. aspersa. According to Abbott, in that case "an aspersa animal was placed in an empty Ennea shell and became atttached."
X-raying the shell would help pick between the alternatives 2 and 3. An X-ray photograph could even reveal hidden cracks and expose the specimen as a fake. All I could do at the museum was hold the shell against a light, but that wasn't helpful.
Note that the bands on the Helix shell are visible thru the translucent Lymnaea shell near the lip of the latter. This is explained (if the whole thing is not a fake) by the fact that when a snail resumes growth after a break, the new shell starts growing below the old shell and slightly behind the break. I have discussed this with examples in 2 previous posts (here and here).
I am not familiar with the name, Cailliaud, on the label. The only Cailliaud in 2,400 Years of Malacology is Frédéric de Nantes Cailliaud (1787-1869). If this specimen indeed belonged to him, then it is from before 1870. Cailliaud might have written about this specimen or others like it. I will appreciate any further information, including literature citations, that anyone might have on Cailliaud's work with chimeras.