Subshrubs. Stems erect, 150–1500 mm, glandular-pubescent to viscid-villous. Leaves cauline, relatively even-sized; petiole absent; blade elliptic-lanceolate or lanceolate to narrowly lanceolate or linear-lanceolate, sometimes narrowly oblong, 20–75(–100) × 4–20(–28) mm, margins entire or shallowly crenate, plane or revolute, apex acute to obtuse, abaxial surfaces moderately villous, hairs unbranched, vitreous, adaxial glabrous. Pedicels 3–5 mm in fruit. Flowers 2 per node, chasmogamous. Calyces not inflated in fruit, 28–40 mm, densely glandular-pubescent to short glandular-villous, tube slightly dilated distally, lobes unequal, apex acute, ribs green, intercostal areas light green. Corollas usually pale yellow or cream to yellow, not spotted or striped, palate ridges yellow to golden yellow, tube-throat 35–42 mm, limb 20–30 mm diam., bilabiate, lobes oblong, apex of adaxial 2 each shallowly, asymmetrically incised. Anthers exserted, glabrous. Styles minutely glandular. Stigmas exserted, lobes equal. Capsules 25–35 mm. 2n = 20.
Phenology: Flowering Apr–Jul.
Habitat: Granite outcrops, boulders, rocky gullies.
Elevation: (300–)700–2200 m.
Although first described as a separate species, Diplacus calycinus has more recently been treated at subspecific or varietal rank (A. L. Grant 1924; F. W. Pennell 1951; P. A. Munz and D. D. Keck 1973). D. M. Thompson (2005) included both D. calycinus and D. longiflorus within his concept of Mimulus aurantiacus var. pubescens (Torrey) D. M. Thompson. He did not reference the study of sect. Diplacus by M. C. Tulig (2000), but results from the Tulig morphometric analyses indicated that D. calycinus is distinct from D. longiflorus, especially in corolla length, corolla tube length, and style length.
The type of Diplacus calycinus is from Tulare County, and the concept of the species is perhaps best restricted to the Sierran population system in Fresno, Kern, and Tulare counties, disjunct from D. longiflorus, which occurs primarily in coastal counties. The Sierran system is characterized by distinct abaxial leaf vestiture; the hairs are unbranched, broad, and vitreous, compared to the branched, thinner, and dull hairs of D. longiflorus. Plants of D. calycinus parapatric with D. longiflorus also show a tendency toward the characteristic vestiture and also have lighter-colored (but more variable in color) corollas with narrower but slightly shorter tubes. Intergradation between D. calycinus and D. longiflorus occurs in the region connecting the San Bernardino and San Gabriel mountains in San Bernardino County.