Episternal---------------------------Episternal |\__ __/| | \__ __/ | | \__ __/ | | \__ Entosternal __/ | | __/ \__ | | __/ \__ | | __/ \__ | |/ \| Hyosternal Hyosternal | | | | | | | | Hyposternal-------------------------Hyposternal | | | | | | | | Xiphisternal------------------------Xiphisternal. The articulations in the tortoise are indicated by the connecting lines. Geoffroy tries to show that the sternum in other animals is composed of these nine bones, or at least of a certain number of them, always in the same invariable relative positions. Thus in birds the sternum consists of five pieces, of a huge keeled entosternal, and of two "annexes" on either side, which are the hyo-and hyposternals. These are separate only in young birds. Occasionally, especially in young birds, rudiments of episternals and xiphisternals also occur. The minuteness of the episternals and the xiphisternals may be attributed to the gigantic size of the entosternal, in accordance with the _Loi de balancement_. In the other air-breathing Vertebrates the nine sternal elements can according to Geoffroy be discovered without great difficulty. But when we come to the determination of the sternum in fishes, difficulties abound, which Geoffroy solves in the following way. He points out that between the clavicles (_cleithra_) and the hyoid bone (_basihyal_) in fishes there is a long median bone (_urohyal_) which is attached in front by two strong tendons to the horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had seen in this bone the homologue of the sternum. Geoffroy adopts this view, but considers that this bone alone cannot represent the whole sternum. He finds the representatives of other bones of the sternum in the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the _ceratohyal_) which are comprised in the hyoid arch. But he is immediately met by the difficulty that this complex of bones is situated in front of the pectoral girdle, whereas the sternum in higher Vertebrates lies behind the pectoral girdle. He reflects, however, that the gills of fish, situated in front of the clavicles, are merely the lungs under another name. The gills have become shifted forward by a metastasis similar to that which brought the whole thoracic organs far forward in fish. This being so, their supporting