THE LATERAL CANAL SYSTEM. 361 



connection between the orbital and the oral ; there is a short series like a 

 rndinientary 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 9'0 cephalic disks. 

 The second species is of a more common type ; the disks are moderate in 

 size, of a nearly uniform development ; there are 60 disks on the head ; 

 there is a short aural branch of two disks on each side ; and tlie cranial 

 gives no sign of a frontal branch. 



Leucicorus lusciosus and Mixoims cmidalis, 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. Insciostts has 58 cephalic disks, the other, 31. caudalis, has 62. 



Dicrolene nigra and Dicrokne filamcntosa, Plate LXXV., 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 Porogadits longiceps and Ilohomycteronus dirjittahis, 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 

 imiforndy 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 04, probably 06, disks on the head. 



Bassozctus nasiis, Plates LXXVII. and LXXVIII., 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 



