422 Edward Phelps Allis jr., 



of the nomenclature heretofore employed hy me, has pushed upward, 

 between the frontal and the squamosal, to the level of the dorsal 

 surface of the skull. The main infraorbital canal here traverses the 

 superficial layers of this bone, but that section of the canal that is 

 enclosed in the bone contains no sense organ, and is not limited at 

 either end by a primary tube. The bone does not accordingly con- 

 tain a fused latero-sensory skeletal unit, and the canal that traverses 

 it must be secondarily enclosed in it. Mc Murrich [53] says that 

 this section of canal is, in embryos of this fish, enclosed in an osseous 

 tube that is wholly independent of, and only later fuses with the sub- 

 jacent perichondrial bone. He does not say whether, or not, this tube 

 is developed wholly independent of and wholly detached from the 

 immediately adjoining squamosal latero-sensory ossicle; but it is quite 

 probable that it is simply an anterior portion of that ossicle that has 

 become detached from it because of its own secondary fusion with 

 the underlying and encroaching sphenotic. But, whatever the ex- 

 planation may be, the bone that here encloses the infraorbital canal 

 is certainly not the homologue of the dermal postfrontals of Amia 

 and Lepidosteus, for it encloses no sense organ, it lies posterior to 

 the point of anastomosis of the main infraorbital canal with the supra- 

 orbital, and it is, as will be shortly seen, almost wholly wanting in 

 Silurus glanis, the larger part of the corresponding section of the 

 canal of this latter fish simply lying in a shallow groove on the 

 dorsal surface of the sphenotic (postorbital ossification) without bony 

 envelope of any kind. 



The homologue, in Ameiurus, of the dermal postfrontal of the 

 bony ganoids thus certainly, in my opinion, not being represented in 

 any part of the so-called postfrontal or sphenotic of the fish, it must, 

 if it be not wholly absent, be looked for in one of the infraorbital 

 ossicles of Mc Murrich's descriptions. The most dorsal infraorbital is, 

 according to that author, an almost square bone; the next anterior 

 one in the chain being long and slightly curved. In Ameiurus melas, 

 also, the dorsal ossicle is a small one, and, according to Herrick, it 

 lodges no sense organ. This small dorsal ossicle is shown in certain 

 of my sketches of Ameiurus nebulosus but not in others; being quite 



