THE STERNUM IN FISHES 59 



great difficulty. But when we come to the determina- 

 tion of the sternum in fishes, difficulties abound, which 

 Geoffrey solves in the following way. He points out that 

 between the clavicles (cleithrd) and the hyoid bone (basihyaT) 

 in fishes there is a long median bone (urohyat) which is 

 attached in front by two strong tendons to the horns of the 

 hyoid and is free behind (see Fig. i). 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 repre- 

 sentatives 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 immedi- 

 ately 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 metas- 

 tasis similar to that which brought the whole thoracic 



^j 



organs far forward in fish. This being so, their supporting 

 elements, the sternum and the ribs, must have moved 

 with them, and are hence to be found in front of the pectoral 

 girdle. 



Geoffrey's next step is to point out that the only possible 

 homologues of sternal ribs are the branchiostegal rays, which 

 arise from the large bones of the hyoid arch. If these are 

 sternal ribs, the bones to which they are attached must be 

 the hyo- and hyposternals or "annexes," the bones from 

 which in birds the ribs take their origin. 



The unpaired sternal bone (urohyal} cannot be homo- 

 logous with the entosternal, for it has no connections with 

 the annexes. He decides that it must represent the episternals, 

 for in some young birds there is a two-headed episternal 

 to which two strong tendons are attached, just in the same 

 way as the unpaired piece in fish is bound to the bones of 

 the hyoid by two tendons. "Thus it is not the sternum as a 

 whole that has shifted in front of the clavicles and covered 

 with its side pieces the gills placed there ; it is a piece 

 exclusively piscine, in the sense that it is only in the class of 



