Heimatomyces Peyr., Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften. Wien . Mathematisch-naturwissenschaftliche Classe. Abteilung 1 68: 251 ( Peyritsch 1873 ) Laboulbeniomycetes (Fungi, Ascomycota) of Denmark Santamaria, Sergi Pedersen, Jan European Journal of Taxonomy 2021 2021-11-26 781 1 425 62LWW 1049342 Peyr. [716,954,1282,1309] Laboulbeniomycetes Laboulbeniaceae Chitonomyces Fungi Laboulbeniales 97 98 Ascomycota genus  MB#1002    Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften. Wien. Mathematischnaturwissenschaftliche Classe. Abteilung 168: 250 ( Peyritsch 1873).–   Typespecies:  C. melanurusPeyr.      HeimatomycesPeyr., Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften.  Wien. Mathematisch-naturwissenschaftliche Classe. Abteilung 168: 251 ( Peyritsch 1873)[MB#2252]    Brief description Monoecious. Receptacle six-celled. The primary appendage, which is subtended by cell IIIa, is two-celled and consists of a dome-shaped basal cell and a filiform distal cell. Only one antheridium for each thallus, usually also dome-shaped and mostly similar to the basal cell of the primary appendage (this corresponds to the genus  Chitonomycesin the strict sense, see below), embedded between cell IIIa and the perithecium.    Remarks The genus consists of 89 species, of which 13 inEurope, seven in Denmark. The most recently described species is  C. tylopus( Santamaria 2001b). This is an isolated and well-characterized genus associated with aquatic beetles, mostly Dytiscidae, but also Haliplidaeand Gyrinidae. Some species show a high degree of specialization by growing on very restricted areas of host bodies ( Goldmann & Weir 2012). Thalli show a bizarre morphology requiring a particular terminology. Thaxter (1924)divided the genus in two subgenera, Euchitonomycesand  Chitonomyces s. str.for species with six or eight receptacular cells, respectively. The only further difference between the two typesseems to be related to the distinctiveness of the antheridium which is, weakly distinguished in Euchitonomyces(e.g.,  C. bidessarius). Otherwise we see no reason to separate the two groups. The six receptacular cells are named I, Ia, II, IIa, III, and IIIa ( Santamaria 2003; Goldmann & Weir 2012) ( Fig. 20B). Because of the absence of a thorough study of thallus development in this genus, the exact identity of some cells has not been clearly interpreted, particularly the cells lying under the perithecium, as m, n, n′ and VII.    Key to the Danish species (partially following Santamaria 2003)     1. Species with some kind of horn-like outgrowth perithecial cell, including basal and stalk cells .... 2  – Species without this kind of outgrowths on or near the perithecium ............................................... 4   2. A perithecial outgrowth on the top, arising from an uppermost wall cell. Thalli growing under the posterior margin of left elytron. On  LaccophilusLeach, 1815(Col. Dytiscidae) ............................... ...............................................................................................................  C. paradoxus(Peyr.) Thaxt.  – Perithecial outgrowth lateral. Thalli on other locations of host body............................................... 3   3. A perithecial outgrowth arising from a preapical wall cell. Thalli growing under the margins of the pronotum. On  GraptodytesSeidlitz, 1887(Col. Dytiscidae) and  HaliplusLatreille, 1802(Col. Haliplidae) .............................................................................................................  C. aculeiferSpeg.  – A perithecial outgrowth arising from the lower third of the anterior side of the perithecium, sometimes also including a bulging cell VI which resembles a second protuberance. Thalli growing on the last tarsus of left hind leg. On  Laccophilus(Col. Dytiscidae) ........................................  C. ensiferSpeg.   4. Cell IIIa proliferating into a lateral outgrowth. On  Laccophilus(Col. Dytiscidae) ............................ ..................................................................................................................................  C. italicusSpeg.  – Cell IIIa not proliferating into an outgrowth .................................................................................... 5   5. Cell IIIa and basal cell of primary appendage blackened and with a hook-shaped tip. On  Laccophilus(Col. Dytiscidae)..................................................................................................  C. melanurusPeyr.  – Cell IIIa and basal cell of primary appendage hyaline to pale yellowish-brown, concolorous with the remaining thallus .............................................................................................................................. 6   6. Cell I very elongate, 5–10 or even more longer as broad. Thalli concealed under the last abdominal sternite of  Laccophilus(Col. Dytiscidae) .............................................................  C. elongatusSpeg.  – Cell I shorter, at most twice longer as broad. On  HygrotusStephens, 1828(Col. Dytiscidae)........... ............................................................................................................  C. bidessarius(Thaxt.) Thaxt.