486 Edward Phelps AUis jr., 



horizontal cheek line of pit organs of Amia, and hence the hyomandi- 

 bular canal of selachians. 



The parietals of Scaphirhynchns are fused to form a single median 

 bone, and this bone lodges four short canals, two on either side, as 

 shown in the figure. These two canals, on either side, are evidently 

 the homologue, together, of the single canal of Acipenser, and, as in 

 the latter fish, they are of smaller calibre than the main canals. 



Adjoining the nuchal plate, and adjoining also the succeeding four 

 plates of the mid -dorsal series of the trunk, this being as far as this 

 was traced, there is, on either side, a row of small plates each lodg- 

 ing a short transverse canal that opens at either end by a single 

 pore. The organs that must be lodged, one in each one of these 

 canals, apparently correspond to the organs of the dorsal body line of 

 pit organs of Amia. 



The nerve sacs are even more numerous in this fish than in Aci- 

 penser, and, as in the latter fish, there are four large and prominent 

 patches of these organs on the dorsal surface of the head, and also a 

 long line of them on the dorso -lateral border of the rostrum. That 

 these groups of organs can not represent any of the pit lines of Amia 

 and teleosts, notwithstanding the somewhat corresponding positions of 

 certain of them, seems conclusively proved by the presence of the 

 several canals of small caliber. 



Polyodon folium. The canals in this fish t have already described, 

 in an earlier work \10\ as fully as the material in my possession 

 permits. In general arrangement the canals difier from those in Aci- 

 penser only in that the canal that I have described, in the latter fish, 

 as the preopercular canal, is a much longer one in Polyodon, and, in 

 this latter fish, anastomoses by its dorsal end with the main infra- 

 orbital. In Polyodon, as in Acipenser, there is an ethmoidal cross- 

 commissure, and at the lateral end of the commissure there is, as in 

 Acipenser, what I at the time called, "an anterior continuation of the 

 canal on either side or the trunk of a dendritic system, whichever it 

 may be". Polyodon thus apparently here agrees exactly with 

 Acipenser. 



Comparison with Acipenser and Scaphirhynclius shows that bone 



