The pathological types of gastroenteritis do not present many unusual features. Simple injection of vessels during digestion seems more evident in this than any other order, while mucus production seems less marked. The mucosa of the reservoir portions may, in simple overfilling and fermentative distention (gastric tympanites), be quite normal or dull red; when active fermentation has occurred it may be digested and peel off. More or less true inflammation as seen in the digestive stomachs is definitely better expressed in the compound ruminant organ than in the simple equine variety where congestion and edematous swelling with hemorrhage form the usual picture. True catarrhal changes both gross and minute, are often well seen and ulcerative lesions are not uncommon; ulcers are unusual in the simple stomach. These statements hold good also for the small intestine. Enteritis affects the duodenum and jejunum more in the Perissodactyla and swine than in the ruminants. Ungulata do not react with hyperplasia of the mural lymphatics as do many other orders, but the swelling of the mesenteric nodes is often noteworthy. As might be expected this is more definite with catarrhal changes and therefore best seen in the ruminants. In the colon, however, the solitary follicles are often quite prominent in simple inflammation. Histologically the greatest changes seen in this order are superficial degenerations with mononuclear increase in the deep submucosa, mostly arranged in perivascular fashion. The villi do not contain the large number of cells seen in the Carnivora. MARSUPIALIA. Consultation of the figures for this order in Table 11 arrests attention at once because of the peculiar percentages found under the detailed factors. Thus food is of no moment as a disturber of the alimentary tract, while bacteria and parasites are high in importance. The gastric segment is more often attacked than any other portion and slightly more often than in the nearest order, Carnivora. These unusual figures deserve explanation, to which purpose it will be necessary to consider the anatomy of the organ and to discuss why bacteria and parasites stand so high in etiology.