THE LATERAL CANAL SYSTEM. 361 
connection between the orbital and the oral; there is a short series like a 
rudimentary spiracular back toward the gill opening, a transverse series 
between the rostrals on the top of the snout, and another transverse series 
between the orals on the chin; the aural and the frontal branch appear to 
have formed a very short loop on each side of the occiput ; each disk rests 
upon a peculiar transverse boat-shaped scale of which each end forms a 
spine or tubercle to protect the organ; and there are 90 cephalic disks. 
The second species is of a more common type; the disks are moderate in 
size, of a nearly uniform development; there are 66 disks on the head, 
there is a short aural branch of two disks on each side; and the cranial 
gives no sign of a frontal branch. 
Leucicorus lusciosus and Mixonus caudalis, Plate LXXIV., are closely allied 
forms, on which the disks are greatly differentiated, postorbitals and spira- 
culars are distinct, a short aural passes up and forward as if to form an 
occipital commissure, and frontal branches are absent. The blinded species, 
L. lusciosus has 58 cephalic disks, the other, IZ. caudalis, has 62. 
Dicrolene nigra and Dicrolene filamentosa, Plate LXXY., illustrate the 
most available differences for the formation of the minor divisions of the 
genera, for instances slight variations in the groupings of the disks on post- 
orbitals and spiraculars, in the interspaces, in the curves of the series, and 
in the sizes of the disks. Aural and frontal branches are similar in the 
number of disks, but differ slightly in directions ; the number of disks on 
the head is 64 in each species. 
In Poregadus longiceps and Holeomycteronus digittatus, Plate LXXVI., a 
long-headed species of one genus is contrasted with a short-headed species 
of another genus. The disks differ in size on the two species, but are 
uniformly developed on each; neither has frontal branches. On the first 
the disks are small, farther apart, there are 68 on the head, the series are 
straighter, and the aural branches contain but two disks each. On the 
second species greater dependence is evidently placed on the system; the 
disks are much larger; the series are more curved; there are three disks in 
each aural branch; and there are 64, probably 66, disks on the head. 
Bassozetus nasus, Plates LXXVII. and LXXVIIL, is a good exemplifica- 
tion of the consequences to the system of function in special directions from 
the body, due to particular habits. The disks have an extraordinary 
development on the upper portions of the head, while on the lower portions 
they are comparatively small. The disks of greatest development are those 
