146 GEORGE E. NICHOLLS 
It was found that the normal dogfish would, after a period of 
activity, settle indifferently upon any part of the aquarium floor 
apparently neither shunning nor choosing the well lighted parts 
of the tank. Whether, however, they came torest upon the 
floor of the tank or upon a rocky ledge in the aquarium, it was 
observed that they almost invariably settled in some position 
which gave room for the body to stretch out freely with the 
tail extended horizontally in the line of the long axis of the 
body. In such a position (figs. 1, 6) the wedge-shaped head lies 
with its ventral surface lifted from the floor, but the long axis 
of the brain has an approximately horizontal position. The 
trunk, from the branchial region almost to the end of the pelvic 
fins, lies slightly flattened ventrally against the supporting sur- 
face. The pectoral fins are disposed nearly horizontally out- 
wards and backwards. The anal fin is bent over, near its base, 
sharply to one side so that the actual ventral surface of the ani- 
mal, behind the pelvic region, is supported just clear of the 
bottom. Behind the anal fin, however, in which region the trunk 
tapers off into the tail, the ventral surface no longer touches the 
bottom but is supported well clear of the tank floor. The 
caudal fin rests upon the bottom so lightly that its flexible ven- 
tral border is searcely bent. In this attitude, which was found 
to be invariably assumed by fish confined in the small tanks, the 
long axis of the central nervous system (which coincides with the 
position of Reissner’s fiber) is maintained, practically, in the hori- 
zontal plane. Only at its hinder end, in the heterocercal tail, 
is this axis slightly upturned. 
In the aquarium, in which an attempt is made to reproduce 
more nearly the natural condition, the bottom is frequently 
uneven. Whether, however, the fish settles upon the roughly 
level floor or perches itself upon some jutting rocky shelf, it 
will be found to maintain the posture described. Upon an 
uneven supporting surface it will be seen that the body bridges 
stiffly the gaps between inequalities of the surface and the tail 
maintains its nearly horizontal position even if there be no 
contactual surface beneath the caudal region. It is not unusual 
to see a dogfish resting with the trunk supported upon a rocky 
