In your example (what is it?) I see hooked crochets, minums perched above the line & semibreves hanging below. One trick when scoring partbooks where one can't assume no typos is to work backwards from the last cadence.

The hooked rest is obviously(?) a crochet rest (quarter note rest).

See the table of rests in the Wikipedia article on Mensural Notation for this and other rest symbols (which closely parallel the modern usage).

I feel like I desperately need someone to teach me to read mensural notation, it's a lacuna of knowledge between mediaeval and modern notation.

I have read articles, and I can (obviously) get the whole thing about basic note values, and ligatures aren't too difficult to get in themselves, though the rules are cumbersome to remember, where I get lost is the whole business of tempus, prolatio, and coloration. Frankly, not knowing this makes me feel highly inadequate...

For anyone interested in Willi Apel's The Notation of Polypphonic Music 900 to 1600, here is a link to the volume: https://archive.org/details/notationofpolyph00apel

I had a look at the first few pages and quickly clicked the "x". I am seriously not ready for this!

Modern rest symbol? ZZZzzzz