The Latero-Sensory Canals and Belated Bones in Fishes. 429 



moid) of my descriptions of Amia. A purely dermal bone that is 

 found in this region in certain other fishes, and there described as a 

 supraorbital, or prefrontal, lies, it should be noted, wholly lateral to 

 the supraorbital canal. The homologue of the strictly primary ant- 

 orbital ossification (lateral ethmoid) of Amia, thus apparently here, in 

 Clarias, comes to the level of the dorsal surface of the skull, and 

 encloses secondarily a section of the supraorbital canal, exactly as 

 the sphenotic (postorbital ossification), in this and other siluroids, comes 

 to the dorsal surface of the skull and secondarily encloses a section 

 of the main infraorbital canal. Posterior to this prefrontal bone, or 

 antorbital ossification (lateral ethmoid), whichever it may be, the canal 

 enters and traverses the frontal, and beyond that bone extends a 

 short distance into the squamosal. It anastomoses, by its 6th. or 

 penultimate primary tube, with the main infraorbital canal, the resul- 

 ting double tube being the one called by Pollard No. 6 infraorbital; 

 the terminal tube of the supraorbital canal being the 7 th. supraorbital 

 tube and not the 6th., as given by Pollard. The frontal bone lodges 

 the posterior four sense organs of the line, no organ lying in the 

 squamosal, the short terminal section of the canal that traverses that 

 bone thus being secondarily enclosed in it. 



The main infraorbital canal first traverses what Pollard describes 

 as a small rudimentary dermal bone, and then in succession what he 

 considers as an antorbital, a lachrymal and a suborbital bone, each 

 of these four bones lodging a single sense organ. The fifth organ lies 

 in a large dermal bone called by both Huxley and Pollard a post- 

 orbital. This latter bone is supraorbital rather than postorbital in 

 position, and is evidently the homologue of the bone that I have 

 identified as the postfrontal in the other siluroids already referred to. 

 That the bone is not here postorbital in position is important, as it 

 shows the bone in a position apparently intermediate between that 

 in Silurus and that in Amia. 



After leaving this postfrontal bone the canal is said to traverse 

 the so-called postfrontal of Huxley and Pollard, and while in that 

 bone it anastomoses with the supraorbital canal. The bone lodges 

 no sense organ and is, accordingly, as in the other siluroids, the 



