In August 2008, I announced here my finding of the snail Discus catskillensis on Mont Royal in Montreal, Canada*. Subsequently, a few readers questioned my identification and suggested that the snail I had found was more likely the European species D. rotundatus. While the former species is a native of North America, the latter was introduced to the eastern U.S. early in the 20th century and later to Canada.
For a more definite identification, I needed more specimens. But I did not have a chance to go back to Mont Royal until August 2011 when I was able to collect a few more shells. Last fall, I compared the Montreal specimens with the shells of several Discus species. The results indicated that the Montreal species was indeed D. rotundatus.
The final outcome of this project was a short paper that recently got published in Check List. You may download a copy of it from here.
*I have now deleted the original post of 2008. Attempting to revise it would have been too much trouble.