The word is reservaculum, "something used to keep things in", from reservo "keep (back)". I believe this word is used to describe the pouch of marsupials in similar texts from that period. Praesumably, this was before the word marsupial was invented, which is derived from Latin marsupium, "pouch".
Incidentally, you have uon where it should be non.
A plain horizontal mark above a letter normally indicates that a nasal consonant (m or n) has been omitted, in Mediaeval or later manuscript, in languages like Dutch, German, or Latin. Alternatively (but not in this text), it can be used as a general mark of abbreviation.
Note that the mark above the first u in reservacŭlum is a different sign. A mark that looks like ˘ or even a short diagonal line or semicircle can be used to indicate that the letter below it is a u or v, in order to avoid confusion with n or other letters.