522 ALLE ES. [VOL. II. 
temporal part of the main canal and in the upper end of the 
operculo-mandibular. The glossopharyngeal, which in Amia 
supplies a canal organ and line of pit organs lying almost 
directly above the end of the opercular canal, is in Fierasfer 
distributed entirely to the first branchial arch, and takes no 
part in the innervation of the lateral system. 
Variations. — The arrangement of the canals and organs of 
the lateral system given in the preceding descriptions is the one 
most commonly found; but there are frequent variations, due 
either to the complete disappearance, or to the addition, of one 
or more peripheral systems. A group of sense-organs and its 
nerve appear or disappear with the addition or disappearance 
of a system. No exception to this rule was found in Amia; 
but in one specimen of Amiurus, a primary tube was missing 
in the mandibular line without the disappearance of a corre- 
sponding nerve and organ, thus leaving two separate organs or 
groups of organs between two consecutive tubes. This was 
doubtless due to the abnormal closing of a tube after its regular 
formation. 
In Amia the most frequent variation from the normal type 
was the disappearance of one sense-organ and corresponding 
primary tube in the mandibular canal, leaving nine organs, 
tubes, and pores along the lower edge of the mandible in- 
stead of ten. The missing system was always one of those 
normally found in the angular. Less frequently there was the 
addition of one system, or the disappearance of two, along this 
same canal, the number along the opercular part of the line 
always remaining constant. 
Other variations were the addition or disappearance of a single 
system along that part of the suborbital canal that lies in the 
postorbital bones, and the addition or disappearance of one in 
the one-half of the supratemporal cross-commissure, the other 
half of the commissure often varying inversely, so that the total 
number of systems in the full commissure remained the same. 
The organ that is missing when there is one system short in 
the suborbital line is either No. 11 or No. 12, both of which 
normally lie in the lower postorbital, and are innervated by a 
single branch of the R. buccalis. No variations were found in 
the supra-orbital line. 
Van Wijhe (No. Io, p. 285) has called attention to the regular 
