220 JOURNAL OF COMPARATIVE NEUROLOGY. 
not to be confounded with the supra-temporal bones lodging 
the supra-temporal cross-commissure of the lateral canals. In 
view of this ambiguous usage it is better to follow STANNIUS 
and call the latter the extra-scapular, avoiding the term supra- 
temporal altogether. A single supra-opercular bone is de- 
scribed and figured for Salmo salar by Brucu (’75), and in the 
next sub-section we shall see that their relations in the Ameri- 
can siluroids are exceedingly various. 
a. The main canal. The main canal runs back for a short 
distance in the frontal bone, then enters the postfrontal and 
squamosal. Although its course in the postfrontal is quite 
long, there is no sense organ contained in this bone, nor is there 
a pore at its union with either the frontal or squamosal, these 
two sutures being very firmly united. In the squamosal seg- 
ment of the canal there are two organs, the first and second of 
the main line, witha long dermal tubule between them, which is 
directed outward and forward and opens by pore / of the main 
canal over the lateral edge of the squamosal bone. In the two 
other specimens of which I have sections the opercular canal 
joins the main canal in the position of this tubule and the pore 
is fused with the last pore of the opercular canal. There is a 
second pore of the main canal behind the squamosal. Then 
follow four ossicles, each containing a single sense organ and all 
separated by dermal tubules and pores. The second ossicle 
clearly is the post-temporal (Fig. 1, PZ) sometimes called the 
supra-scapula and termed supra-clavicula by McMurricu (84, 
p. 301), JAQuET, (98, p. 137) and JuGE (’99),—presenting the 
characteristic articulations with the cranium and with the verte- 
bral column as described by McMourricu. 
The first one of these ossicles is a slender scale-like bone, 
closely wedged in between the post-temporal and the squamosal 
and ca rying the canal along its entire length. It is minute but 
well ossified with a cancellous structure (Fig. 1, ESC.). This 
bone is not mentioned in McMurricu’s account of the skeleton 
of Ameiurus (’84), nor in any other work on siluroid osteology 
accessible to me, and its interpretation is a matter of some diff- 
culty. It appears in all of my specimens of Ameiurus melas 
