Cheebee

Hmm, yes you are somewhat right. I'm near the end of Prime 1 on Trilogy right now, and the only scans I still need to get are the Ice Shriekbats and Metroid Prime's 2 forms. Went to Ice Ruins West, but no Ice Shriekbats. Darn it all. No regular ones either though, just a big Sheegoth.

I think in the PAL version, the shriekbats actually don't disappear. http://metroid.wikia.com/wiki/List_of_Metroid_Prime_Version_Differences