482 AUGUSTA LAMONT ON 



lying in contact with one another as far as their course persists, except towards the 

 extremity of each, where it diverges from the rest. The canals of a sub-group are 

 thus separated by spaces from the next sub-group, but not from one another. This 

 arrangement, and the fact of the exceptionally fine calibre of the canals, as well as 

 their large numbers, suggests the possibility that each such sub-group may in reality 

 be the equivalent of the larger and separately placed canal in the corresponding- 

 group of non-pigmented species. 



(2) Non-pigmented Types. 



The arrangement of the canals in the non-pigmented species is similar to that of 

 the pigmented types, but their distribution is more restricted and their numbers are 

 considerably less. Owing to the absence of pigment in the canals of these species, 

 and also on account of the colours of the skin of the dorsal surface, the dorsal canals 

 are practically indistinguishable before dissection, and their openings are detected 

 only with difficulty. On the ventral surface, however, which in all these species is 

 white, their course can often be traced through the skin, and the openings stand 

 out distinctly as round pores dotted over the surface and predominating in 

 the head region. 



On the dorsal surface the arrangement of the canals is more characterised by 

 restriction in numbers than in distribution. On the ventral surface the canals are 

 both fewer and of less extent, spreading only over the head region and over the basal 

 and anterior portions of the pectoral fin (Plate III, figs. 3 and 4). 



Particular instances of a difference in the manner in which the canals are disposed 

 may be observed in the case of those arising from the ophthalmic and the mandibular 

 ampullae. The dorsal ophthalmic canals, instead of terminating as described for 

 Raia batis at varying distances along the side of the rostrum, are all of approxi- 

 mately equal length ; an anterior fascicle directed forwards has its openings grouped 

 close to the tip of the snout, while the canals of the posterior fascicle, directed back- 

 wards, all terminate close together, anterior to the front angle of the eye. A similar 

 difference is observable in the case of the mandibular canals which, being also more 

 equal in length and running closely parallel towards the median line, avoid the 

 fringe-like arrangement described for It. batis. 



The general arrangement of the canals is better understood by a glance at the 

 accompanying figures than by means of any description, but it must be borne in 

 mind that the drawings do not claim to be exact representations as regards numbers of 

 canals where these are very numerous, deeply embedded, or overlapping one another. 

 The general distribution, and an approximation to the numbers visible is all that has 

 been aimed at, for instance, in the case of the ventral surface of the snout region, 

 where the clear colourless appearance both of the canals and of the gelatinous tissue in 

 which they are embedded renders the former very inconspicuous. In the case of the 

 dorsal posterior hyoid canals, on the other hand, the drawings represent the actual 



