No. 3.] LATERAL LINE OF AMIA. 527 
it and to organ 21. In Fig. 27 the canal has closed over organ 
20 and is about to close over organ 21, the process being so 
greatly abbreviated that pore 21 is formed at once in place. 
The two halves of pore 20 are always formed at some distance 
apart, as shown in Figs. 26, 27, and 28. 
In Fig. 26 the two half-pores that go to form pore 17 infra- 
orbital are some distance apart. In Fig. 27 they are nearing 
each other, and in Fig. 30 they have united. The pore so 
formed is also beginning, in this figure, to travel down the oper- 
cular canal toward the terminal pore 17, which is formed by the 
closing of the canal over organ 16. In Fig: 33 the double pore 
17-17 is nearly formed, and in Fig. 34 it has undergone its first 
division. 
The formation of pores 18 and 1g infra-orbital is somewhat 
irregular. Organs 2 and 3 of the supratemporal commissure 
are usually the first ones inclosed, but in Fig. 27 the canal has 
formed over organs I and 3, and not over organ 2. The lateral 
terminal opening of the commissure, pore 4, lies directly oppo- 
site organ 18 infra-orbital, and a little in front of and behind 
this organ are the anterior and posterior halves of pores 18 and 
19. In Figs. 28 and 35, which represent the same specimen, 
the canal is beginning to close over organ 18, the upper lip of 
the canal forming at the end of the commissure directly above 
its terminal opening. In Figs. 29 and 36, which also represent 
the same specimen, the canal has nearly closed over organ 18, 
and in Figs. 30 and 31 the process is completed, the terminal 
opening of the commissure being entirely shut off from the 
exterior, —transferred, in fact, from the exterior to the inner wall 
of the infra-orbital canal. In two of the specimens examined 
by section there was an extra pore and tube at this point, both 
small, the tube leading forward and medianward from the point 
where the commissure joined the main canal. 
In Fig. 34 pores 18 infra-orbital and 3 supra-orbital have 
undergone their first division. In this figure the median termi- 
nal openings of the half-commissure of each side have united 
and undergone their first division. This division frequently takes 
place before the two terminal pores have fully united, and in 
such cases the median pore never gets beyond the oblong shape 
shown in Figs. 32 and 33. The same thing often occurs in 
pore 17 infra-orbital, which may undergo its first division before 
