This rare species was growing here in alkali meadow habitat along the littoral zone adjacent to Fish Slough Creek (seasonally wet...though superficially dry at the time of this June visit). The soil surface was covered with an often thick, crunchy, alkali crust of white precipitated salts with a very high albedo.

Various local botanists had opined that the June date here was probably too late and hot for any C. excavatus to be found still in flower...but Ceal Klinger held out hope, and led me to these :-)

Here are some references of varying detail: Greene's original 1890 description; Davidson's 1915 description of the synonymous C. campestris; Ownbey's 1940 description; the FNA description; the Jepson eFlora description; and...on the 13th page of the PDF here...a discussion by the late, great, Calochorto-philes Vic Girard & Stan Farwig. The excellent 2007 book by Gerritson & Parsons is, as always, quite good here.

The keys in Ownbey(1940) and the Jepson eFlora get me to C. excavatus here, though the FNA key doesn't quite...due to the lack of an evident green longitudinal stripe on the outer faces of the petals in the plants I photographed.