No. 3.] LATERAL LINE OF AMIA. 469 
The system of canals leading from the pores of any one 
group forms, as already stated, a dendritic structure, the trunk 
of which is the common connection of the group with some 
one of the main canals of the head. These main canals lie 
almost exclusively in the deeper, more porous layer of the 
dermal bones, near their under surface, and often project 
in the form of a ridge below the general level of this surface. 
Bridge (No. 4, pp. 607 and 608) considers this part of the bone in 
Amia an “adherent parostosis,” resulting from the later ossifi- 
cation of the subcutaneous tissues lying immediately under the 
original ganoid plate, which is represented in the superficial, 
thinner, and denser layer of the bone. The development, how- 
ever, shows that the dermal bones grow mainly by accretions to 
their upper surfaces, and that the deeper portion is the one 
formed first, the canals in young specimens lying in it or above 
it, and in no case below it. 
In their passage from one bone to another, where the bones 
are not suturally connected, the canals lie in a dense connective 
tissue, which forms the deeper part of the cutis. This occurs 
particularly between the frontal and nasal on either side, and 
between the upper and lower ends of “he pre-operculum and the 
squamosal and angular respectively. 
The trunks of most of the dendritic systems and the proxi- 
mal parts of their branches lie in the dermal bones, but in their 
upper denser layer. The openings of the branches on the upper 
surface of the bone present all the various stages between the 
single opening and two openings about to become separate that 
the pores do on the external surface of the head. The number 
of openings on the upper surface of the bone does not, however, 
usually correspond with the number of external pores; for where 
the dermis is thick, as on the end of the nose, around the pos- 
terior nares, and along the edges of the pre-operculum, there 
may be one or more series of branches lying in it, entirely above 
or beyond the bones; and where the dermis is thin, although 
some of the branches pass directly through it to the external 
surface, others, of the creeper-like form shown in Cut 1, %, are 
continued along the outer surface of the bone in long channelled 
lines, the bone not yet fully inclosing them. 
