"an t-slige mhòr"
Translation:the big shell
The same reason why there’s no /s/ pronounced in an t-sùil gheal – it historically was lenited to /h/ and then disappeared, but instead the old /t/ of the article is visible here.
I’ve also written about it in the discussion below an t-slige ghorm.
In general, always when you see a word starting with t-s… in Gaelic, the s is not pronounced, it’s replaced by the t.