Basionym. Lichen exiguus Ach., Lich. suec. Prod. 69 (1798).
Type. without locality (BM-ACH - lectotype, Ropin & Mayrhofer, Herzogia 9: 800, 1993).

Exsiccatae. Arnold Lich. Exs. 1381, 1654 (MIN, latter as R. ramulicola); Kerner Flora Exs. Austro Hungar. 2749 (NY); Malme Lich. Suec. Exs. 234, 908 (COL); Plantae Graecenses, Lich. 104 (COL); Rabenhorst Lich. Eur. Exs. 453 (COL, MICH); Vězda Lich. Bohem. 239 (COL, COLO); Wartmann, Schenk & Winther Schweiz. Krypt. 263 (BM, SASK as R. metabolica var. exigua).

Description. Thallus thin, light to dark grey, areolate; areoles usually discrete at first, sometimes sublobate, becoming rimose-areolate, to 0.50-1.00 mm wide; surface plane, becoming rugose to minutely verrucose, matt; margin determinate; prothallus lacking; vegetative propagules absent. Apothecia broadly attached to slightly narrowly attached, frequent, becoming contiguous, angular by compression, to 0.55-0.90(-1.20) mm in diam.; disc black, plane to convex; thalline margin concolourous with thallus, entire or rarely incomplete, 0.05-0.10 mm wide, persistent; excipular ring absent.  Apothecial Anatomy. Thalline exciple (45-)60-90 µm wide laterally; cortex indistinct, 5-10 µm wide; epinecral layer mostly lacking; crystals present in cortex, absent in medulla; cortical cells rarely pigmented, to 5.0-6.0(-7.5) µm wide; algal cells to 14.0-17.5(-24.0) µm long; thalline exciple (60-)80-120 µm deep below; cortex absent or 10-25 µm wide, poorly organized; proper exciple hyaline, indistinct, 5-10 µm wide laterally, widening to 10-30 µm peripherally; hypothecium hyaline, 60-100(140) µm deep; hymenium:70-100 µm high, not inspersed; paraphyses 2.0-2.5 µm wide, not conglutinate, apices to 3.0-5.0 µm wide, darkly pigmented, forming a brown to dark red-brown epihymenium; asci 50-70 x 15-21 µm. Ascospores 8/ascus, Type A development, Physcia-type, (15.5-)18.5-19.0(-22.0) x (7.5-)8.5-9.5(-10.5) µm, average l/b ratio 2.0-2.2, tending to become more acute at maturity, sometimes slightly inflated at septum, not more so in KOH, lumina becoming inflated, apical walls mostly remaining thick; torus prominent at maturity; walls darkly pigmented, quite strongly ornamented. Pycnidia not observed.

Chemistry. Spot tests, K+ yellow or K-, C-, KC-, P+ faint yellow or P-; secondary metabolites, atranorin in cortex.

Substrate and Ecology. Corticolous, found on Abies concolor, Fraxinus, Quercus agrifolia and Umbellularia californica, at elevations of 550-1 660 m. The species has been collected with R. aurantiaca, R. hallii, R. laevigata, R. orculata and R. oregana.

Distribution. In North America R. exigua appears to be limited to California, where it is  infrequent in the Coastal Ranges and Sierra Nevada. Known elsewhere in the world from Europe, north Africa and Australasia (Mayrhofer and Moberg 2002). Ryan 24832b (accompanying the type of R. aurantiaca), reported as R. capensis by Mayrhofer and Sheard (2002), belongs to R. exigua.

Notes. Rinodina exigua is characterized by its Physcia-type spores, cortical atranorin, indistinct cortex, large areoles and rugose thallus. In dark grey thalli, atranorin is present in low concentrations but is always visible under polarized light (with one known exception from Scotland) as previously noted by Mayrhofer and Moberg (2002). Some spores may be slightly inflated at the septum but are unchanged after the application of KOH. It is distinguished from R. capensis by the lack of an expanded, columnar lower cortex and its more acute spores at maturity with more strongly ornamented walls. Nevertheless, young thalli with plane areoles may have a very similar habit to R. capensis and it is possible that some specimens have been misidentified as R. capensis. In contrast to R. capensis, the spores of R. exigua in North America are larger than those quoted for Europe (Fox and Purvis 1992, Giralt and Mayrhofer 1994).

Rinodina californiensis has the same chemistry and spores of a similar length, but broader (average l/b ratio 1.7-1.8), which belong to the Dirinaria-type and inflate at the septum in KOH. This species is primarily coastal but also occurs in the Cascade Range and therefore might be mistaken for R. exigua. The Rocky Mountain species R. boulderensis, also possesses cortical atranorin and Physcia-type spores which are shorter (averaging 16.0-17.0 µm long) and lack wall ornamentation. 

Specimens examined. U.S.A. CALIFORNIA. Eldorado Co., 2 mi E Kyburz, S.C. Tucker 34017, 34110b; 3 mi E Kyburz, 1995, S.C. Tucker (all SBBG); Los Angeles Co., Angeles Nat. Forest, B.D. Ryan 26209; Mount Mooney Road, B.D. Ryan 26214a; San Gabriel Wilderness, B.D. Ryan 26475b; Madera Co., Rock Creek, B.D. Ryan 32166b (all ASU); Mariposa Co., Yosemite Valley, B. McCune 28142 (personal herb.); Monterey Co., Hastings Natural History Reserve, S.C. Tucker 34586d; Nacimiento, C.C. Bratt 6207 (both SBBG); Riverside Co., San Jacinto Mts., C.M. Wetmore 14689 (SASK); Santa Barbara Co., Santa Ynez Mountains, S.C. Tucker 10981 (LSU); Siskiyou Co., 2.5 mi SW Etna, B.D. Ryan 24832 (ASU); Tulare Co., 1 mi NE Colony Peak, C.M. Wetmore 51303; 2 mi NE Lookout Point, C.M. Wetmore 50160; Ash Mountain, C.M. Wetmore 50865; Buckeye Flat Campground, C.M. Wetmore 50308; Potwisha Campground, C.M. Wetmore 50890; Shepherd Peak, C.M. Wetmore 50299; South Fork Ranger Station, C.M. Wetmore 50821 (all MIN).

Selected References. Sheard (1967 Fig. 5C), Fox & Purvis (1992), Ropin & Mayrhofer (1993 Abb. 3), Giralt & Mayrhofer (1994a Fig. 2B), Giralt (2001 Plate XVI: E), Mayrhofer & Moberg (2002 p. 103).

Global occurrence: Africa (incl Madagascar) | Eurasia – Europe | Americas – North America (incl Mexico) | Arctic.  Substrate: wood – dead, living | bark, cork, plant surface – trunks, branches, twigs.  Life habit: lichenized (mutualistic with algal photobionts). Thallus: crustose (crustaceous) – episubstratal – unspecified; continuous, diffuse, effuse | cracked, fissured, fractured, rimose; [th] upper surface: grey(ish) | white(ish) | white(ish) brown (beige); [th upper surface]: epruinose; [th marginal and upper surface] specific structures: absent; [th] morphol substructures (eg areoles, lobes, branches) upper surface: smooth, plane | granulose, granular.  Ascomata: absent | present; ascoma: apothecial, apothecioid – hymenial; ascoma [mm]: (low) 0.3 (high) 0.6 (max) 0.7; ascoma: immersed, innate | subsessile, subimmersed, adnate, semi-immersed, emergent | sessile, superficial; [ascm, if apoth] disc, mazaedium: plane, flat, flattened, expanded; [ascm, if apoth] disc, mazaedium: black(ish); [ascm, if apoth] disc, mazaedium: epruinose; [ascm, if apoth] margin surface; [if perith] periostiolar area, ostiole, involucrellum: grey(ish) | white(ish) | white(ish) brown (beige); [ascm, if apoth] margin excipular photobionts: absent | present; [ascm, if apoth] subhymenial layers, hypothecium; [if perith] basal excipulum: hyaline, colourless; [ascm] paraphyses/-oids: present; [ascm] epihymenium, epithecium: brown(ish) (if pale: fawn, tan; if mid: cinnamon) | grey(ish) brown | yellow(ish) brown | red(dish) brown (pale: orange brown).  Asci: lecanoralean; [asc] tholus: thickened; [asc] tholus amyloidity (iodine reaction): present; [asc] tholus amyloidity pattern: amyloid with widening axial body towards the apex (= Lecanora-, Parmelia-, Rinodina-types etc).  Ascospores: (median) 8.0; [asp] shape: ellipsoidal; [asp] length [µm]: (low) 15.0 (high) 21.0 (max) 23.0; [asp] width [µm]: (low) 7.0 (high) 9.0 (max) 10.0; [asp] septa: present; [asp] septa: transversely septate; [asp] transversal septa: (median) 1.0; [asp] pigmentation: pale brown | medium brown, brownish | dark brown; [asp] perispore, epispore: ornamented, rough, warted, striate, cracked.  Secondary metabolites: present, atranorin.  Primary photobiont: present, chlorophytaceous – trebouxiaceous, chlorococcoid.  Secondary photobionts (eg in cephalodia): absent.