40 F. L. LANDACRE 
of the epidermis at the point where the pharyngeal endodermic 
pocket joins the ectoderm, (c) the anterior extension of the audi- 
tory vesicle (the preauditory placode of Ameiurus, Landacre, 
’10, the branchial sense organ of Wilson, ’91), (d) the thickening 
of the epidermis extending caudad from the point of origin of the 
epibranchial placode. 
In all the epibranchial placodes there is a thickening of the 
epidermis at the point where the endodermic gill pocket joins the 
ectoderm and this always lies anterior to the point of origin of the 
placode. The epibranchial placode always arises at the poste- 
rior endof this thickening and while in Lepidosteus the placodes are 
unusually long in their transverse axis they are thin from anterior 
to posterior and abut against the posterior border of the ecto- 
dermic gill invagination. In all the placodes except the VII, 
and sometimes here, there is a sharp color differentiation as well 
as histological differentiation between the two structures and one 
can be quite sure of this distinction in the later stages of the 
placode, although the two structures are continuous, when one 
reads the sections from the ectodermie gill shelf into the placode. 
The primordia of the lateral sensory lines can be differentiated 
from the epibranchial placodes with equal ease except in the case 
of the VII. They lie at a different level, much above the placodes 
in IX and X, but in VII the supra-orbital, sub-orbital and man- 
dibular sensgry lines converge at the hyoid gill rendering it diffi- 
cult to differentiate between primordia of lateral sensory lines 
and the early stage of the epibranchial placode. Much the same 
difficulty exists in the VII with reference to the preauditory pla- 
code. This structure in its anterior extension drops to the level 
of the hyoid gill and seems in the earlier stages, before the appear- 
ance of the epibranchial placode, to become continuous with the 
ectodermic gill thickening in this region although it changes 
its histological characters at the anterior end. To add to these 
difficulties in the case of the VII, each epibranchial placode in 
Lepidosteus is continued caudad by a thickening of the epider- 
mis, which in the case of the IX and X ganglia lies at a lower 
level than the fundament of the lateral lines and of course in these 
cases cannot be confused with the dorso-lateral placodes. 
