patur – I am not famillier with what pliny wrote, i will try to look it up. However, once you are coming on to Pliny and assume he is talking about techeiles than there is no way he is not talking about the murex. ( and generally speaking [without sparking a huge hashkafa debate] we only listen to these chachmei umos haolam as long as they are not being soiser chachmei yisrael like the radvaz)
About what you say that the simanim should really tell us it is a snail. I am assuming that the word Chilazon literally means snail therefore the gemara will not tell us it is a snail.
?? ???? ?? ???”? (????? ? ?) [?????? ????? ????? ??? ??”? ?????? “?????” ???? “????”,[1] ??”?: “??? ?????? ??? ???? ????? ??? ??? ????? ???? ????? ??? ?? ???? ???? ???”, ???”? (????? ?? ?, ?????? ??.) ????? ???? ?????”? ?????? ?? ?????[2] ?? (??? ??:, ???”? ?:)];
??? ??’ ??”? ????? (?”? ??:) “????? ??????: ???”? ???”?”[3]
??? ???? ????”? (??”?, ????? ?”? ?, ?????? ????? ??”? ?????? “?????” ??? “?????”).
[1] ?????? ????? ?????? ??”? ?”? ?? ???? ??? ????? ?? ???? ???? ????? ??????, ?? ????? ?? ?????? ??????? ???”?.
[2] ???’ ??????? ??? (??’ ?? ?) ?????? ????? ????? ??????.
[3] ?”? ??? ?????? ?? (????? ?’), ??”? ???? ????? ???? (??? ?????) ?”? ?”? ????; ?????? ???”? ??? ???? “????”, ?????? ?”?????” ????? ???????, ???? ?”?????” ????? ??????.
And by the way it is page 52 of the kuntress but it is page 58 of the pdf file. and the picture in which the shell is white the caption says clearly that it is a picture after it dries up