Methanobacteriumthermophilum is the correct name if this species is regarded as a separate species (i.e., if its nomenclatural type is not assigned to another species whose name is validly published, legitimate and not rejected and has priority) within a separate genus Methanobacterium.
On the Validation List no. 34 the reference of the paper by Laurinavichus et al. is incorrectly cited as Mikrobiologiya, 1987 (sic), 57, 1035-1041.Publication:
Wasserfallen A, Nolling J, Pfister P, Reeve J, Conway de Macario E. Phylogenetic analysis of 18 thermophilic Methanobacterium isolates supports the proposals to create a new genus, Methanothermobacter gen. nov., and to reclassify several isolates in three species, Methanothermobacter thermautotrophicus comb. nov., Methanothermobacter wolfeii comb. nov., and Methanothermobacter marburgensis sp. nov. Int J Syst Evol Microbiol 2000; 50:43-53.
Boone DR. Genus IV. Methanothermobacter Wasserfallen, Nölling, Pfister, Reeve and Conway de Macario 2000, 51VP. In: Boone DR, Castenholz RW, Garrity GM (eds), Bergey's Manual of Systematic Bacteriology, second edition, vol. 1 (The Archaea and the deeply branching and phototrophic Bacteria), Springer-Verlag, New York, 2001, p. 230-233.