54(53) Spores ellipsoid, sometimes inequilaterally flattened, 44-54 × 22-30µm, with 1 apical germ pore, caudae not swelling in water. Perithecium usually with lateral tufts of agglutinated hairs up to 550µm long. _Arnium arizonense_ - Spores evenly ellipsoid-fusiform, 31-55 × 18-25µm, with germ pore at each end, caudae covering germ pores, 35-60 × 7-11µm, but rupturing and swelling to up to 130 × 50µm, and becoming diffuse and irregular. Perithecial neck covered with rigid hairs up to 190 × 2.5µm. _Arnium hirtum_ 55(53) Perithecial neck distinctly setose with rigid hairs. 56 - Perithecial neck without setae. 57 56(55) Spores evenly ellipsoid-fusiform, 31-55 × 18-25µm, with germ pore at each end, caudae covering germ pores, 35-60 × 7-11µm, but rupturing and swelling up to 130 × 50µm, and becoming diffuse and irregular. Perithecial neck covered with rigid hairs up to 190 × 2.5µm. _Arnium hirtum_ - Spores slightly inequilateral, 35-43 × 17-23µm, caudae 50-75 × 5-8µm, not covering germ pores. Perithecial neck with brown hairs up to 250µm long. _Arnium cervinum_ 57(55) Perithecia covered with a dense tomentum of septate flexuous hairs. Spores mostly longer than 45µm. Only occasionally fimicolous. 58 - Perithecia without a tomentum. Spores up to 45µm. 59 58(57) Spores (40)45-54 × 25-35µm, uniseriate. Tomentum pale or grayish. _Arnium olerum_ - Spores 47-70 x 20-30µm, biseriate above. Tomentum olivaceous brown. _Arnium tomentosum_ 59(57) Spores somewhat inequilateral, rounded below, pointed above, 31-40 × 18-24µm, caudae 50-120 × 6-10µm, with 1 apical germ pore not covered by cauda. _Arnium caballinum_ - Spores equilateral, 36-44 × 20-23µm, caudae 50-80 × 6-8µm, covering germ pores. _Arnium mendax_ 60(51) Perithecia with scales at the neck, composed of inflated and agglutinated cells (fig. 27, _S. conicum_). (_Schizothecium_) 61 - Perithecia setose or hairy at the neck, but not with inflated cells, or neck black but almost hairless. (_Podospora_) 70