64 CARL L. HUBBS 



are separate (as they usually are). The plates of the opercular 

 series in the isospondylous fishes differ from those of Amia in the 

 following respects: the reduction of the suboperculum, so that the^ 

 interoperculum and operculum are in contact anteriorly;^ the 

 proximal (or anterior) attachment of branchioperculm and branchi- 

 opercular fold to the hyoid arch; the more complete imbrication 

 of all the rays; the attachment of branchiostegals to the epihyal 

 as well as to the ceratohyal; the frequent reduction of the rays 

 below the main hyoid suture to rather slender rods, and the occa- 

 sional attachment of these reduced rays to the edge of the cera- 

 tohyal, rather than to its outer face. These last two features are 

 apparently caused by the strong development of the musculus 

 geniohyoideus of the lower jaw, which is attached to the hyoid 

 arch near the suture separating the ceratohyal from the epihyal. 

 The number of the larger and flatter rays attached to the outer 

 surface of the epihyal (the lowermost sometimes on the suture) 

 varies widely in the Isospondyli and related orders; the writer has 

 counted one in Bathylagus pacificus; two in Pterothrissus gissu, 

 Hiodon tergisus, Osmerus thaleichthys, Osmerus attenuatus, Arius 

 gagora, and Amiurus nebulosus; three in Amphiodon alveoides 

 Ethmidium maculatum/ Alepocephalus agassizii, Coilia ectenes, 

 and Hypomesus olidus; either three or four in Albula vulpes; four 

 in Chirocentrus dorab, Salvelinus fontinalis, Osmerus mordax, 

 Mallotus villosus, Plagyodus ferox, Lestidiops sphyraenopsis, 

 Bathysaurus ferox, Chlorophthalmus chalybeius, and Neosco- 

 pelus macrolepidotus; five in Etrumeus micropus, Felichthys 

 felis, and Dalha pectoraUs; six in Oncorhynchus nerka, Saurida 

 gracilis, and Bathypterois pectoralis; either six or seven in Trachi- 

 nocephalus myops; seven in Esox lucius, Aulopus japonicus, and 

 Synodus intermedius; eight in Esox americanus and Synodus 



3 According to Woodward's restoration of the primitive extinct genera Lep- 

 tolepis and Holcolepis, these genera bridge the gap between Amia and Elops in 

 the character of the opercula. These bones apparently show no significant 

 variation among living teleosts, though certain ones are reduced or increased in 

 size in certain genera; the interoperculum is especially liable to variation, being 

 occasionally absent. 



* All but the lowermost of the six on the ceratohyal are of similar shape to 

 those of the epihyal in Ethmidium maculatum. 



