This is a land snail that Thomas Say described twice, first in 1817, and then in 1821. In his first description1, Say named it Helix lineata.
Only four years later, he described it again2, but this time placing it in the freshwater snail genus Planorbis. Say had found it in a dry pond bed and, therefore, but as his remark indicates, reluctantly, considered it an aquatic species.
It is odd that Say didn’t recognize his second specimen as being the same as his earlier Helix lineata. Or, did he? His descriptions for the two species are almost the same and their dimensions are identical. In both cases he even picked specific names, lineata and parallelus that presumably refer to the characteristic spiral lines clearly visible on the shell. I suspect that Say was aware that the two species were similar, if not identical, but the location where he had found the species the second time, “a dried up pond”, confused him. Perhaps, he thought that he was dealing with two similar looking species, one terrestrial and the other aquatic.
The shape of the shell of this snail, now called Helicodiscus parallelus, is unique among North American land snails3. It comes very close to being planispiral, meaning a shell coiled in a plane and lacking the elevated spire that we familiarly associate with snails shells.
The shells of the freshwater snail family Planorbidae are also planispiral. That is why Say picked the planorbid genus Planorbis in his second description. According to Raup & Stanley4, gastropods with planispiral shells were common in the Paleozoic. But, for unknown reasons such shells are generally rare among extant gastropods.
Side view of a planorbid shell (left) and its cross section (right), showing the protoconch in the center.
1. Say, T. 1817. Journal of the Academy of Natural Sciences of Philadelphia, 1:17-18.
2. Say, T. 1821. Journal of the Academy of Natural Sciences of Philadelphia, 2:149-179.
3. There are a few other species of Helicodiscus that also have planispiral shells.
4. Raup, D.M. & Stanley, S.M. 1978. Principles of Paleontology. 2nd ed.