No. 3.] LATERAL LINE OF AMIA. 505 
epithelium of Exoccetus and other forms. The groups which 
thus arise are doubtless the beginnings of what Merkel has 
called nerve-ridges (No. 12, p. 20), for under this name he in- 
cludes all organs found in the adult inside the canals of the 
lateral system (No. 12, p. 23). He also finds nerve-ridges on 
the surface in some fishes that have no canals, as Petromyzon, 
Squatina, etc., and describes them as simply an elongated form 
of the regular conical nerve-hillock, containing exactly the same 
histological elements, but with a larger proportion of support- 
cells. He further says, that in the teleosts the conical hillocks 
are always found on the surface or in slight depressions ; that 
they may either be arranged in lines and groups, or scattered 
irregularly over the whole body; that they are most numerous 
in those forms that have but few openings to the lateral canals, 
‘or have no canals at all; and that where the canals branch 
freely, and there are many pores, the conical hillocks may 
entirely disappear, as on the head of Mullus. In the Sela- 
chians they are, according to him, found only on two species, 
Mustelus and Squatina, being entirely replaced in other forms 
by another kind of sense-organ, the nerve-ampulla (No. 12, p. 39) ; 
and in the Ganoids, he says, they are not found at all, being 
entirely replaced by still another kind of organ, the nerve-sack 
(No. 12, p. 36). This certainly is not true of all the Ganoids, 
for in Amia I have not been able to find any of the nerve-sacks 
which he considers peculiar to the Ganoids, and I have always 
found seven different lines of pit organs (his conical hillocks) 
on each side of the head, and two series of lines of organs on 
each side of the body. 
Of the seven lines found on the head, three lie on top of it, 
two on the cheek, one on the side of the mandible, and one on the 
gular plate (Figs. 21, 22, and 23, Pls. XXXVI. and XXXVII_). 
Of the two on the body, one lies parallel to the lateral line, and 
the other on the top of the body close to the dorsal fin. Each 
of the lines on the head is a continuous series of organs, while 
those on the body in the adult are each made up of a series 
of such lines, one approximately to each segment of the body. 
No ventral body line could be found. 
a. Head Lines. — The first or anterior one (Fig. 21, a/) of 
the three dorsal lines of the head runs backward and median- 
ward from group 8 supra-orbital; and the third or posterior one 
