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Show 348 MESSES. F. E. BEDDAED AND P. CHALMERS MITCHELL [May 7, be obvious (see fig. 2). As will be seen by an inspection of the accompanying drawing (fig. 3), which w e are enabled to reproduce here by the courtesy of Prof. Lankester, this half of the valve is bound down anteriorly to the outer wall of the ventricle by a strong muscular bridge. This muscular bridge is also connected with the anterior edge of another valve which is much shorter than the one just described, but which runs approximately in the same direction as the last, i. e. nearly parallel to the longitudinal axis of the heart. This has generally been accepted as part of the longer valve and as not corresponding to the septal flap. W e are, however, unable to agree with this interpretation of the structure. In its relations it corresponds exactly to tbe septal flap of the valve of the Alligator; the upper end of the two valves in the Alligator are in the same way bound down by a strong band of muscle ; it is true that this muscle does not form a definite bridge, but it stands out in relief, and as the spongy wall of the ventricle is so much thicker, w e cannot regard the obliteration of the space beneath as of any morphological significance : in fact w e identify what has been called the inner part of the valve in the Bird's heart as the septal flap of the valve of the Alligator's heart. It is identical in relative position, in its mode of attachment; and in some birds w e have seen a slight development of tendon in its substance. Furthermore, the direction of the muscular fibres is not continuous round what has been regarded as the continuous edge of the valve. As to its disproportion in size, w7e do not see that the amount of development as compared with the nature of the development is a point of much significance. The left auriculo-ventricular valve, illustrated in the drawing (fig. 4), consists of two separate vertical flaps, of which the septal is considerably larger and overlaps the left flap; the septal valve is thinner, and is composed of both muscles and tendon. The left half of the valve has tbe free crescentic edge strongly ligamentous, the remaining part being muscular. The left aorta arises of course from the right ventricle; its exit is guarded by three watch-pocket valves, on the free edge of which are cartilaginous hardenings ; the exit is narrow, and the aorta then dilates into a wide chamber in the bulbus. In this are two rows of small sculpturings like rudimentary valves extending to the top of the wide part of the aorta, each row being vertically above the middle of one of the valves (fig. 2, a, b). The pulmonary artery arises from the same ventricle; its exit is similarly guarded by three valves aud it similarly dilates into a wide expansion iu the bulbus ; the pulmonary artery at the end of the dilatation divides into two branches, which, however, leave the bulbus on its ventral aspect and not, as figured in Wiedersheim, on the dorsal aspect. The right aorta arises from the left ventricle ; its narrow exit is guarded by two wide valves, behind the left of which arises the coronary artery. Behind the right lies the very large communication with the left aorta. The aorta then expands into a very wide sac in the bulbus ; in a line with the communication between |