THE LATERAL SENSE ORGANS OF ELASMOBRANCHS. 481 



advocate the separation of the pigmented species into a distinct genus under the 

 name of " Batis." 



This grouping, arrived at by Couch from external characters, and confirmed by 

 dissection of the ampullary canal system, would, as he himself points out, be in 

 accordance with the distinction made by fishermen, who, in Devonshire at all events, 

 call Raia batis a " skate," while they class R. fullonica, R. clavata, R. maculata, 

 R. microcellata, and R. circularis all together as " rays." 



(l) Pigmented Types. 



Raia batis, Linn., or the common skate, taken as the only available representative 

 of the types with pigmented canals, is easily distinguished from any species of the 

 non-pigmented group. Contrasted with these it is characterised by the presence of 

 a dark pigment in the canal walls at and towards their openings. The pigment 

 causes the openings of the canals to appear on the external surface as little black 

 dots, and also makes the course of the canal visible as a grey streak for a short 

 distance through the skin. 



The large number and wide distribution of the canals are somewhat less pro- 

 nounced on the dorsal surface (Plate I). The canals of the posterior hyoid group 

 usually number ten or eleven, and are of varying lengths. It is convenient to take 

 special notice of this group for purposes of comparison, its isolated and superficial 

 position, as well as the small number, greater length, and larger calibre of its 

 component canals, rendering accurate counts and correct delineation easier than in 

 other cases. Another characteristic feature on the dorsal surface occurs in the 

 distribution of the canals of the ophthalmic group of ampullae ; these, as already 

 described, lie parallel to the rostrum, one group running forwards to the tip of the 

 snout, and another backwards to the nasal capsule ; but in this case only canals of 

 varying lengths terminate between the two extremities, so that their openings are 

 roughly grouped in linear series along the edge of the rostrum. 



On the ventral surface (Plate II) the canals are so numerous and so widely 

 dispersed as to defy enumeration, covering almost the whole of the cephalic, thoracic, 

 and abdominal regions, as well as the whole surface of the pectoral fin, with the 

 exception of a zone about 15 inch wide (in specimen 17 to 18 inches across) along 

 the posterior edge. The canals of all the three anterior groups of ampullae are 

 widely divergent, as are also those of the inner hyoid group, while the mandibular 

 canals, although directed inwards, are not, as in other cases, closely parallel, but 

 diverge somewhat and terminate at varying distances posterior to the lower jaw, 

 forming an elongate fringe-like arrangement extending from the corner of the 

 mouth inwards. 



It is, however, in regard to the outer group of the ventral hyoid canals that the 

 most distinctive grouping is observable. The canals here are clustered in little sub- 

 groups of about six or seven, those forming a group being of different lengths, but 



