468 ALLIS. [Vou. Il. 
single instance was found of such a growth from below, and 
but one of the reverse condition,—that of a tube leading 
blindly inward from the external surface toward a canal; and 
the conditions in this case seemed to indicate that the tube, 
after its regular formation, had been closed secondarily below 
the surface. The specimen in which this was found was taken 
in winter, through the ice, and having been frozen and fre- 
quently handled, the epidermis was so injured before the pit 
was discovered that it was impossible to tell whether there had 
originally been a regular surface pore or not. 
The formation of the more or less complicated groups of 
pores resulting from this method of division is well shown in Fig. 
17, Pl. XXXV., which represents the head of an Amia about 
fifteen months old and 114 inches long. This specimen, which 
was raised in an aquarium, was much larger than other speci- 
mens two years old living in the same aquarium, and probably 
larger than fishes of the same age found in their native waters ; 
for, according to Dr. Estes (No. 7, p. 659), Amia one year old, 
taken in the sloughs tributary to Lake Pepin, are only from 
three to six inches long. All stages in the division of a pore 
are shown in this figure. In some instances the pore is appar- 
ently dividing into three portions instead of two, but this is due 
to the accelerated re-division of one of the portions before the 
regular division has been completed. Nothing of this kind was 
found in any of the several adult fishes examined, but in the 
skeletons prepared, the openings of the canals on the upper sur- 
face of the frontal often presented a tri-lobate appearance, indi- 
cating that they had arisen in this manner (Pl. XL., Fig. 40, 
Ce ca) 
The different groups of pores, although varying greatly in 
size and shape in different individuals, or even in the same 
individual on opposite sides of the head, are normally definite 
in number and general position. Each group when small is con- 
fined strictly to some particular region of the head; but in its 
growth it extends beyond this region, and becomes continuous 
with neighboring groups. It is necessary, therefore, in order 
to arrive at the proper number and arrangement of the groups 
in large and well-developed specimens, to trace the canal lead- 
ing from each pore to the particular trunk or stem from which 
it arises. 
