Gudger (1930) ends an article on this deformity by stating that double-mouth deformity in fishes is always due to injury so far as our present knowledge goes. The feature common to all cases is displacement of the lower end of the hyoid arch downwards and backwards through a gap in the mouth floor. This occurs because the protractor muscles of the arch have been put out of action, usually by severe accidental injury. The retractor muscles, thus left unopposed, pull the lower end of the arch into the deformed position. It is difficult to imagine how accidental wound could produce the condition seen in the Shetland trout. Here is only a cleft like a clean cut in the midline of the mouth floor ending at the displaced arch. There is neither irregular scarring nor obvious tissue loss, the head retaining its bilateral symmetry. In it the protractors of the arch must also have been destroyed, but probably by some other kind of damage than wound from without. With this possibility in mind I wrote to the managers of several trout-farms, enclosing a sketch of the deformity. Each was asked if he had come across anything like it when grading his stock. One letter in the affirmative suggested that air trapped in the floor of the mouth might be the cause. Later the writer sent me two young trout, radiographs of which confirmed this statement and led me to think that obstruction of the pneumatic duct might account for the trapped air. The consequences of extirpation of the swimbladder in physostomes explains how the trapped air is permanently at increased pressure, and supports this hypothesis.