Fungi classified in the Mytilinidiaceae Kirschst. (Kirschstein 1924) are characterized by persistent, fragile, carbonaceous ascomata, which range from globoid to obovoid to strongly laterally compressed, erect oyster-shaped (conchate) or hatchet-shaped (dolabrate) structures, standing on edge, usually with a prominent longitudinal keel or cristate apex (Fig. 1; Barr 1990; Zogg 1962). Mytilinidiaceous fungi possess a thin-walled, prosenchymatous peridium enclosing a hamathecium of narrow trabeculate pseudoparaphyses, borne in a gel matrix, which are often sparse to lacking at maturity (Barr 1987). Bitunicate asci are borne in a basal, rarely lateral, orientation within the centrum, and contain eight, rarely four, ascospores, overlapping uniseriate, biseriate or in one or two fascicles. Ascospores are diverse in the Mytilinidiaceae and range from scolecospores to didymospores to phragmospores to dictyospores, hyaline, soon yellow to dark brown in pigmentation, and generally showing bipolar symmetry (Barr 1990). Currently accepted genera (Eriksson 2006) in the Mytilinidiaceae include Actidium Fr., Lophium Fr., Mytilinidion Duby, Ostreichnion Duby, Ostreola Darker, and Quasiconcha Barr & Blackw., to which has recently been added Zoggium Vassiljeva (Barr 1975, 1990; Barr & Blackwell 1980; Darker 1963; Lohman 1932; Vasilyeva 2001; Zogg 1962) (Table 1). The genus Glyphium, with erect dolabrate ascomata, was originally included by Zogg (1962), Barr (1987, 1990) and others (e.g., Goree 1974; Lorenzo and Messuti 2005; Sutton 1970) in the Mytilinidiaceae. However, recent molecular evidence, based only on a single isolate of G. elatum (Grev.) Zogg (CBS 268.34), using the nuclear small (nuSSU) and large (nuLSU) ribosomal subunits, as well as the mitochondrial small (mtSSU) and large (mtLSU) ribosomal subunits (Lindemuth et al. 2001; Lumbsch et al. 2005), has removed the genus Glyphium to the Chaetothyriales in the Eurotiomycetes (Geiser et al. 2006; Lucking et al. 2004; Schmitt et al. 2005). Anamorphic states in the Mytilinidiaceae are primarily coelomycetous (e.g., Aposphaeria, Pyrenochaeta, Camaroglobulus, Dothiorella and Sclerochaeta) and less frequently hyphomycetous (e.g., Chalara-like, Papulospora, Peyronelia and Septonema) (Lohman 1932, 1933a & b; Speer 1986; Sutton 1970). Typically temperate in distribution, mytilinidiaceous fungi are found in association with the wood, bark, resin, cones, scales, needles, seeds, and roots of gymnosperms and are less commonly recovered from angiosperms (e.g., Ostreichnion) (Barr 1975, 1990; Zogg 1962).

Barr (1987): “Ascomata superficial or with bases immersed in substrate, conchate (bivalve shell shaped) or dolabrate (axe-head shaped), somewhat elongate, sometimes depressed shield shaped but ellipsoid from above, medium to large sized; apex typically cristate, opening by longitudinal slit; peridium carbonaceous, brittle, prosenchymatous, at times composed of cephalothecoid plates. Hamathecium of sparse traveculate pseudoparaphyses, at times lacking at maturity. Asci bitunicate, basal, cylindric or clavate, with narrow ocular chamber. Ascospores hyaline soon yellowish to light or dark brown, broadly filiform, one or several transversely septate, murifom in some; wall smooth or ornamented; contents usually with one globule in each cell; overlapping uniseriate, biseriate or in one or two fascicles in the ascus. Anamorphs coelomycetous where known (Aposphaeria, Pyrenochaeta, Sclerochaeta) or infrequently hyphomycetous (Chalara-like, Peyronelia, Septonema). Saprobic, occasionally hemibiotrophic, on leaves, branches, cones of gymnosperms, some on woody angiosperms, cosmopolitan. The conchate or dolabrate ascomata with thin brittle peridium & crested apices are representative of the family. Segregated from the Hysteriales as the Lophiaceae by Zogg (1962), who described & illustrated many of the taxa. Lohman (1932, 1933) described several species of Mytilidion* & other taxa. Darker (1963) described Ostreola, which now contains O. formosa, a common species in western North America. Glyphium was monographed by Goree (1974); Sutton (1970) described its hyphomycetous Anamorphs. Barr (1975) enlarged Ostreichnion & separated three species. Barr & Blackwell (1980) erected the genus Quasiconcha for a fungus found on Juniperus seeds in dung; Q. reticulate is now known to occur on conifer roots (Blackwell & Gilbertson 1985). Considerable variability is evident in size, shape & sepatation of ascospores within the family, characteristics that permit separation of genera in the following key.”

Barr (1990): “Ascomata superficial or bases immersed in substrate & widely erumpent, separate or gregarious, conchate or dolabrate, somewhat to strongly elongate, sometimes depressed & scutate; apex typically cristate (crested), occasionally low inconspicuous ridge, opening by longitudinal slit; surface shining, often longitudinally striate; peridium carbonaceous, brittle, prosenchymatous, at times forming cephalothecoid plates. Hamathecium of trabeculae, often sparse or at times lacking at maturity, in gel matrix. Asci bitunicate, basal to lateral, cylindric, clavate or oblong. Ascospores yellowish to light or dark brown, variable in shape & sepatation; walls smooth; one or more globules per cell; uniseriate, biseriate or in fascicle in the ascus. Anamorphs coelomycetous or infrequently hyphomycetous where know. Saprobic on gymnospermous substrates typically, some on woody angiosperms. Notes: Hawkswoth & Eriksson (1988) have proposed the conservation of Lophiaceae over Mytilinidiaceae. The only reservation that I have with this proposal is that the Testudinacae (von Arx, 1971) may provide yet another family name. The members of the Testudinaceae have rounded cleistothecioid ascomata that lack the typical cristate apex opening by a longitudinal slit. The connection lies in the congruity of ascospores between Testudina terrestris Bizzozero & Quasiconcha reticulata Barr & Blackwell, as suggested by Barr & Blackwell (1980). The taxa of the Mytilinidiaceae present some striking ascomata, culminating in the ligulate to dolabrate ones in species of Glyphium. The family is well separated from the Hysteriaceae of the Pleosporales in shape, structure of perdium & cristate apex, as pointed out by Zogg (1962) & others. Von Hohnel (1918) was one fo the earliest to recognize two groups of genera in the old Hysteriaceae. The outermost layers of peridium form radiating rows of rectangular cells, much as in the taxa of the Microthyriaceae. The trabeculae are not conspicuous & are often lacking in mature ascomata.”

1. Ascospores one septate, small, less than 30 μm long.

= go to 2.

1. Ascospores one septate & larger than 30 μm long or several septate.

= go to 3.

2. Ascospores broad, reticulate

= Quasiconcha Barr & Blackwell

2. Ascospores narrow, smooth or finally longitudinally striate

= Actidium Fr. 

3. Ascospore filiform, multiseptate, about equal in length to that of the ascus

= go to 4.

3. Ascospores ellipsoid, fusoid, cylindric, not equal in length to that of the ascus

= go to 5.

4. Ascomata conchate

= Lophium Fr.

4. Ascomata dolabrate

= Glyphium Nitschke ex Lehmann

5. Ascospores transversely septate (if more than 50 μm long only 2-4 μm wide)

= Mytilinidion Duby 

5. Ascospores muriform (or large & remaining one septate)

= go to 6.

6. Ascomata conchate; ascospores ellipsoid, not over 30x10 μm, with single longitudinal septum in mid-cell

= Ostreola Darker

6. Ascomata conchate or approaching dolabrate; ascospores ellipsoid or cylindric, more than 30 μm long, with several longitudinal septa in cells or large & remaining one-septate

= Ostreichnion Duby