_Differential Diagnosis._ From _psorospermosis_ (_coccidiosis_) it is distinguished by its origin on the mucous membranes, and not on the skin, the skin lesion being a secondary one. In psorospermosis the primary lesion is usually on the skin, from which it extends to the mouth and especially along its floor. In psorospermosis the morbid deposit assumes the form of rounded warty-like masses, on comb or wattles; is easily propagated by inoculation, is promptly checked by antiseptics, does not tend to produce internal extension nor generalization, and on microscopic examination shows numerous spheroidal coccidia intermingled with the epidermic cells and possessing amœboid movement. By virtue of this automatic movement they make their way between and into the epidermic cells in which they multiply. From the _croupous angina_ of Rivolta it is distinguished by the absence of the infusoria (monocercomonas gallinæ) to which he, Delprato and Pfeiffer attributed that affection. The monocercomonas is a flagellate organism 14 _μ_ to 25 _μ_ in length and 5 _μ_ to 7 _μ_ in breadth. Its rounded end bears one flagellum as long as the body, and its acute end three flagella which give it active motions. These are found in the yellowish white swellings of the mucosa, which vary in size from a millet seed to a pea, covering a hyperæmic spot and composed of epithelial cells, blood globules—white and red,—leucocytes, granules and the infusoria. The false membrane is remarkable for its lack of consistency and its tendency to invade the mouth and gullet rather than the air passages. These infusoria are not colored by picrocarminate of ammonia, but stain by methyl-violet and then appear as round or slightly irregular hyaline bodies. From _aspergillus disease of pigeons_, by the absence of the characteristic, miliary, white nodule of that disease showing caseated contents intermixed with an abundant mycelium of aspergillus fumigatus. The aspergillus disease attacks especially the mouth but may also implicate the gullet, lungs, liver, intestine and kidneys. The microscopic examination of the exudate is conclusive, by reason of the presence of the bacillus diphtheriæ columbarum, and the comparative absence of the filamentous mycelium.