Most radiologists must have been puzzled at some time or other, when examining radiograms of the chest, by the occasional finding of a fine convex line which begins at the right apex and curves downwards and inwards towards the mediastinum, to end just below the level of the costal cartilage of the first rib in a dense comma-shaped shadow. The situation of both the line and the comma-shaped shadow is, however, subject to wide variation. In some cases the line is quite near to the mediastinum, in others much further out and the comma-shaped shadow may be as low as the level of the costal cartilage of the second rib. (Figs. 1 and 2.) One of us (J.H.M.) when reporting, has drawn the attention of his physician colleagues to this shadow, without hitherto being able to offer any adequate explanation of its presence.In July of this year, Bendick and Wessler (1928) described and interpreted this shadow, and pointed out how few are the references to it in Röntgenological literature. Wessler and J aches had described it for the first time in 1923; they thought that possibly it was due to some congenital anomaly, because of the constancy of its appearance in relation to the right upper lobe. Velde (1927) suggested that the convex line might represent the fissure which cuts off the so-called azygos lobe from the rest of the upper lobe of the right lung, an explanation supported by Hjelms and Hultén (April 1928).