On certain features of the lateral canals and cranial bones of Polyodon folium. 669 



than with B^. The bone was found in two pieces in two of the dis- 

 sections, the smaller or additional one of the two pieces in each case 

 enclosing a section of the supraorbital canal. The posterior one or 

 two of the several ossicles that together form bone 3 lie directly upon 

 the dorsal surface of bone 1 and bone B\ while the one or two 

 anterior ossicles bridge the interspace between B'^ and bone 2, resting 

 at either end upon the dorsal surfaces of these two bones. In one 

 dissection two long and delicate spicules of bone extended mesially 

 from the bridge of bone and reached and rested upon the dorsal 

 surface of the lateral edge of bone B^; a similar spicule extending 

 laterally from this latter bone and resting upon the ventral surface 

 of the ossicles of the bridge. In another specimen a similar spicule 

 extended laterally from bone B^ and reached and rested upon the 

 ventral surface of the ossicles of the bridge. 



The main infraorbital canal, when it leaves the anterior end of 

 the chain of squamosal ossicles, enters the bone that I have called 

 bone 1 not far from its hind edge, and from there runs forward to 

 the middle and thickest part of the bone. There the single canal 

 separates into two parts, one turning downward and forward as the 

 postorbital part of the main infraorbital, while the other runs mesially 

 and forward as the supraorbital canal; each of these canals soon 

 leaving bone 1 to enter, respectively, bones 2 and 3. 



Bone 1 thus encloses a Y-shaped portion of the lateral canals of 

 the head, taken at and around the point where the supraorbital and 

 main infraorbital canals anastomose, the former canal quite certainly 

 here anastomosing with the latter by a terminal and not by a penult- 

 imate primary tube. From the Y-shaped portion of canal thus en- 

 closed in this bone certain tubules always arise, but their number and 

 position varies greatly. Normally a double dendritic system should 

 be found at or near the point of anastomosis of the two canals, and 

 one to three branching tubules did there arise in all but one of the 

 several specimens examined. In nearly every instance there was a 

 tubule at or near the point where each of the three arms of the Y 

 entered or left the bone, these tubules either arising from the canal 

 directly between the two adjoining bones, or near that point but en- 

 closed in one or the other of the two adjoining bones. The tubules 

 thus seemed to indicate that there was at least one sense organ in 

 each arm of the Y, but unfortunately, the sense organs themselves 

 could not be positively recognized at any place in any of the canals. 



43* 



