On certain features of the lateral canals and cranial bones of Polyodon folium. 675 



Bones 1^ 2 and 5 of my descriptions now remain to be con- 

 sidered, and, as already stated, they are capable of two different inter- 

 pretations. The one that first suggests itself is, that they are re- 

 spectively the squamosal, postfrontal, and frontal; and, considered as 

 such, the bones on the top of the head of Polyodon would conform 

 closely with the similarly named bones in Gegenbaur's (10) and 

 Huxley's (13) figures of the cranial bones of Acipenser sturio; 

 Huxley's bone B, shown on one side only of Gegenbaur's figure, 

 being considered as the extrascapular, and his bones I and L as the 

 suprascapular and supraclavicular respectively. The hyomandibular 

 canal of Folyodon would, however, then join the main infraorbital in 

 the postfrontal bone, and the infraorbital and supraorbital canals 

 would anastomose in the squamosal, this latter bone even probably 

 containing, though I cannot positively assert this, a sense organ 

 anterior to the point of anastomosis of the two canals. These re- 

 lations of tbis bone to the lateral canals would be most unusual, as 

 would also be the wide separation of the postfrontal from the frontal 

 by the intervening squamosal. There would also be the several lateral 

 canal ossicles between bone B^ and the extrascapular to account for, 

 and it would have to be assumed either that they represent an un- 

 fused part of the squamosal or such a part of the extrascapular, which 

 latter bone would then have a most unusual anterior extension. 



Under ray second interpretation, which I consider much the more 

 probable, the point of departure is the assumption that the two or 

 more tubular ossicles that lie anterior to the T-shaped extrascapular 

 alone represent the lateral sensory element of the squamosal. My 

 bone 1 then becomes the postfrontal, and it seems much more proper 

 that a postfrontal should acquire the relations that this bone has to 

 the point of anastomosis of the infra- and supraorbital canals than 

 that a squamosal should acquire it. It is, however, contrary to all pre- 

 cedent that a postfrontal bone should lodge either a supraorbital organ, 

 or an infraorbital organ that lies posterior to the point of anastomosis of 

 that canal with the supraorbital. If the bone lodges these several organs, 

 as seems probable, it has, however considered, an unusual relation to the 

 lateral canals, this certainly showing how carefully these canals must 

 be considered in using them to definitely determine the homologies of 

 the related bones. If the bone is nevertheless a postfroutal, my 

 bone 2 necessarily becomes a postorbital, bone 3 remaining the frontal, 

 as under the first interpretation ; the frontal and postfrontal then 

 having proper relations to each other. The hyomandibular canal 



