308 DEEP SEA FISHES. 
branches unite on the occiput, and the majority of the other types figured 
below which possess disunited aurals and a frontal cranial branch that tends 
toward the formation of a loop, as on Lamprogrammus, Plate XXXIV. fig. 3, 
and Plate LXXXI. fig. 1. Amia has the occipital commissure of the aural 
branches; it also possesses the frontal branch backward from the cranial 
toward the aural. For the arrangement and innervation of the system on 
Amia see the work of Allis, 1889, fig. 49. On the body of the teleostean 
the system varies from the complete, extending along the body from head 
to tail, to partial or entire absence, to interrupted series, or to several 
duplicate rows, and from lines of simple nerve papille to those of exces- 
sively modified disks, and the drafts upon the vagus vary accordingly. 
Turning attention especially to the cephalic organs a large amount of 
variation in the numbers of disks will be at once apparent; on the species 
sketched the range is from fifty disks up to ninety. Further than this, 
many species have disks of different sizes or of different degrees of develop- 
ment on different parts of the head. This diversity is a consequent of par- 
ticular habits ; species on which the function of the disks is equally important 
in all directions have the organs about equally developed on the top of the 
head on the sides and beneath, as Mixonus, Porogadus, Bassogigas, and 
Cataetyx of Plates LXXIV., LXXVI., and LXXX.; but others on which the 
function upward accords better with the habits have large disks on the upper 
parts of the head and small ones on the lower, as Bassozetus nasus, Plate 
LXXVII. and species of Eretmichthys, Plate LXXIX.; and still others as 
the Halosauroids, Plate LX XXIV. fig. 1, find a function downward more sat- 
isfying to their necessities and possess disks-of extraordinary development 
on the lower portions of the head and the body while those of the upper 
parts have suffered from neglect. Some of the species have the disks hid- 
den by darkly pigmented mantles from all directions except below; this is 
particularly the case on species of the subgenus Halosauropsis. A wide 
range of perfection in the system is to be seen on the species of Halosaurus: 
on forms like ZZ. attenuatus, Plate LX. fig. 1, the disks and their envelopes 
are so thin as to be almost invisible and so delicately attached to the sur- 
faces of the scales as to be carried away by a very slight rub; on ZH. radia- 
tus Plate LX. fig. 2,a much greater degree of advancement obtains; and on 
H. macrochir and H. rostratus the mechanism appears to have reached the 
extreme of differentiation. On these last the disks and the particular scale 
on which each is seated are much enlarged and the dark mantle by which 
