In all snakes, the Boidae and Xenopeltidae excepted, only the right lung is well developed, while the left lung is rudimentary or absent (BUTLER, 1895). The right lung consists of an anterior alveolar part that is strongly vascularized, and of a posterior smooth-walled air-sac that is anangious. Between these two parts a transitional zone may be present, in which the wall of the lung shows a faint reticulate pattern, and which receives some very fine branches from the pulmonary vessels. In a number of snakes, among which the Viperidae, the situation becomes more complicated. In these snakes the membrane that connects the dorsal ends of the incomplete tracheal cartilages has become greatly expanded, and this dorsal wall has developed an alveolar structure. COPE (1894, p. 218) very aptly has named this the tracheal lung. When the tracheal lung has very strongly developed, it sometimes merges gradually into the right lung. In species with a rudimentary left lung, its opening into the trachea may be considered to mark the end of the trachea, and consequently also the beginning of the right lung. In other species a slight change in the structure of the alveoles may mark the boundary, but in a number of species it becomes a more or less arbitrary procedure to draw a boundary between the tracheal lung and the right lung. For the purpose of the present note it suffices, however, to consider as right lung that part of the respiratory tract that lies posterior to the middle of the heart. The development of the tracheal lung, and the relative size of the alveolar part of the right lung and of the air-sac vary according to genera and species.