The pleura is a tissue apparently quite susceptible to infection in mammals and so closely associated with the air sacs in birds as to be a part of the same membrane, therefore the two being affected together. Throughout the higher class all orders give copious examples of the involvement of the pleura, principally of course as an accompaniment or a sequel to pneumonitic or bronchitic processes but also as a part of acute infectious diseases, such as hemorrhagic septicemia, pleuropneumonia and the like. However two orders present such a number of instances of pleuritis that they deserve notice. The seals, Pinnipedia, of which we have twenty autopsy records, showed inflammation of this membrane four times, three of which were dependent upon pulmonary infection and one apparently due to general septicemia with trifling damage to the lung proper. One of the first cases had gone on to empyema of the classical type, a shrivelled dry almost carnified lung with a thick fibrinopurulent covering. The lung of the seal is well divided into lobules, the external surface being generously supplied with lymphatic channels under the pleura, an arrangement which should carry away infection one would think. Perhaps this high percentage of pleurisy in our Pinnipedia is but accidental. The marsupials, while having a notable percentage of pleurisy both among all the cases and in relation to the number of postmortems, are not so striking from the etiological standpoint since practically all of these have suffered with Kangaroo mycosis or pneumonia. In over half the cases of this infectious disease some grade of pleuritic exudate has been observed, only one, however, going to the stage of empyema.