Okay, I hope
@mcvansoest weighs in here if he can. I would of course appreciate corrections where appropriate. For now here's a few nuggets relating to identification. Again, I would refer you to a helpful section in Starr's book called "Comparing lookalikes" with images and details.
These are the small to medium agaves that tend to get confused in my experience, and it's not always straightforward to distinguish them.
1. isthmensis, potatorum, guadalajarana
2. parviflora, polianthiflora, toumeyana v. bella
3. vic-reg., nickelsiae, and there's a new species in that group I can't remember off hand
4. schidigera, filifera, multifilifera, geminiflora
5. striata, stricta, geminiflora, petrophila
6. parryi, parrasana, havardiana
7. some marginatae (lecheguilla, univittata, funkiana, etc.)
Crudely labeled this way:
1 butterfly agaves
2. small, dark green, shredding margins
3. the royal agaves, with great leaf markings
4. longer leafed shredders
5. hedgehogs (long skinny leaves)
6. blue-green artichoke-ish plants
7. related plants, some used for fiber, which tend to vary and have intermediate forms
Resolving them might work like this:
1a vs. 1b isthmensis is smaller and offsets, potatorum is larger and more likely to be solitary
1c guadalajarana develops pretty distinctive spines when it's all grown up, but younger plants can be tricky to tell from the others
And I would add a caution about three things:
1. Potted plants may look very different from plants in the ground, especially mature plants. This effect is less evident with smaller plants where pot size is not limiting.
2. Very young plants are extremely hard to identify, pretty much only through experience.
3. The plants in cultivation include some very odd variants which are not typical of plants in nature, so habitat photos will only get you so far in identifying cultivated plants, and vice versa. Applying a Latin name to many dwarf agave cultivars is a matter of guesswork.
