"an t-slige bheag"
Translation:the small shell
The s gets replaced by t when lenited after the feminine article. It’s just a weird lenition pattern of s after the article an. You can read more about it in the discussion for "an t-slige ghorm" and "an t-sùil gheal", and on Gramadach na Gaeilge (website about Irish grammar, but the explanation works identically for Scottish Gaelic).
TLDR is that t-s… in Gaelic is always read just as /t/.