Iris ser. Longipetalae is a series of the genus Iris, in Iris subg. Limniris.
William Rickatson Dykes in his book 'Handbook of Garden Irises' (of 1924) includes 4 species in his Iris longipetala subsection; including Iris longipetala (Herbert), Iris missouriensis Nuttall, Iris arizonica (Dykes, 1917) and Iris montana (Nuttall). Iris arizonica and Iris montana have since been classified as synonyms of Iris missouriensis.
The series was then first classified as a 'series' by Diels in 'Die Natürlichen Pflanzenfamilien' (Edited by H. G. A. Engler and K. Prantl) in 1930. It was further expanded by Lawrence in Gentes Herb (written in Dutch) in 1953.
There is still a lot of confusion within the series.
The British Iris Society only lists Iris missouriensis and classes Iris longipetala as a variant of Iris missouriensis.
The American Iris Society and Pacific Iris Society lists three species; Iris longipetala (Herbert), Iris missouriensis (Nuttall) and Iris pariensis (Welsh). But Plant List regards Iris pariensis as a synonym of Iris missouriensis.
The series has species are native to western North America,
They prefer to have moisture in the spring and a dry period during the summer. They also do not like root disturbance hence they are difficult to grow as nursery plants. They are also rarely grown in the UK.