CHAPTER XIII 



PHTHINOBRANCHII: HEMIBRANCHII, LOPHO- 

 BRANCHII, AND HYPOSTOMIDES 



UBORDER Hemibranchii. Still another transitional 

 group, the Hemibranchii, is composed of spiny- 

 rayed fishes with abdominal ventrals. In this sub- 

 order there are other points of divergence, though none of 

 high importance. In these fishes the bones of the shoulder-girdle 

 are somewhat distorted, the supraclavicle reduced or wanting, 

 and the gill structures somewhat degenerate. The presence 

 of bones called interclavicles or infraclavicles, below and behind 

 the clavicle, has been supposed to characterize the order of 

 Hemibranchii. But this character has very slight importance. 

 In two families, Macrorhamphosida and Centriscida, the inter- 

 clavicles are absent altogether. In the Fistulariidcz they are 

 very large. According to the studies of Mr. Edwin C. Starks, 



FIG. 180. FIG. 181. 



FIG. ISO. Shoulder-girdle of a Stickleback, Gasterosteus aculeatus Linnaeus. 

 (After Parker.) 



FIG. 181. Shoulder-girdle of Fistularia petimba Lacepede, showing greatly ex- 

 tended interclavicle, the surface ossified. 



the bone in question is not a true infraclavicle. It is not identical 

 with the infraclavicle of the Ganoids, but it is only a backward 



extension of the hypocoracoid, there being no suture between 



227 



