"the big shell"
Translation:an t-slige mhòr
It is because slige is a feminine noun. The t- before s here is a special counterpart to the lenition of the first consonant in the nominative singular of other feminine nouns, as in e.g. a' bhò 'the cow', a' chrùbag 'the crab'. It occurs after the definite article an whenever this causes such lenition (note that a' in the other examples is a reduced form of an).
In case you're one for historical explanations: In the past the article was ind with a -d at the end. Originally, the initial s- of the following noun was lenited to an h-sound as usual (cf. = /h/), but the combination -d h- eventually became simplified to -t-. So (slightly anachronistically) ind-hlige became in-t-lige, which is nowadays spelled as an t-slige.