Similar clematis would be C. flamula, C. recta, C. finetiana, and C. armandii. I'm not sure how to tell them apart.

It is definitely a clematis but none of the above (with the possible exception of flammula).

It is much more likely to be either C. vitalba or virginiana, if not terniflora, but the best way to distinguish between them is to see the foliage. Can you post a photo showing any clear leaf detail?

Have you sniffed it? C. terniflora has a strong sweet scent. I don’t notice any odor on C. virginiana. (Can’t speak to the others.)

C. virginiana has a similar sweetly scented fragrance when in flower as does terniflora. And due to the same bloom time and similar flowers, they are easily confused from a distance. It is the difference in their foliage appearance that really sets them apart.

Some of the other Clematis mentioned are also fragrant. I didn't include virginiana, because the small 🤏 amount of foliage, that I can see is smooth and not serrated, and virginiana is done blooming here in Illinois, so probably Texas also. All the other species we've mentioned have foliage very similar to terniflora. Even the native leather flower Clematis have similar foliage. I know terniflora gets the white chevron on its leaves, but I don't know if it's the only Clematis that does that?

"All the other species we've mentioned have foliage very similar to terniflora."

Not so much :-) C. recta is fully herbaceous and does not climb....looks more like a shrub and acts like a herbaceous perennial. And armandii is evergreen with long, leathery and very unclematis like foliage and blooms in March, not September. I'd also eliminate both flammula and finetiana, as both are very uncommon in cultivation in the US.

If growing in the wild or an uncultivated area, it is either going to be the native virginiana (bloom time anywhere from end of July to end of October depending on location and siting) or the invasive terniflora or vitalba.

Gardengal, is terniflora also invasive in the Pacific Northwest?

Not that I have seen or seen reported. It flowers too late in our cool summers to set viable seed. I grew one for years in my old garden with never an issue other than its hugely rampant growth habit. Don't have room for such a monster in my current tiny garden :-)