THE LATERAL SENSE ORGANS OF ELASMOBRANCHS. 483 



numbers present in the specimens, since the size, position, and arrangement of these 

 canals renders exact delineation possible. 



With regard to the individual species of this group it is difficult to single out 

 any definite features by means of which they may be distinguished from one 

 another ; but attention may be drawn to one or two points which appear to constitute 

 the most noticeable differences. These occur in the number, length, and divergence 

 of the dorsal posterior group of hyoid canals, and in the length of the most posterior 

 canal or two of the dorsal outer hyoid group. Differences as regards these points 

 will therefore be noted as each species is passed in review. 



Another feature of the species of this group is the absence of a capsular wall 

 surrounding the ampullae of the ophthalmic and inner and outer buccal groups 

 (text-figs. 8 and 9). H. Muller remarked in this connection : " The central organs 

 (i.e. groups of ampullae) are frequently, but not always, surrounded by a strong- 

 fibrous capsule, which is pierced alike by the tubes and nerves." In specimens 

 dissected to make observations on this point, the ampullae were found to be merely 

 embedded in the gelatinous tissue, but a more fibrous layer of connective tissue 

 immediately underlay the skin, covering all the gelatinous area of the snout, and 

 this had to be cut through before the ampullae were exposed. This fibrous layer 

 did not, however, surround the groups of ampullae. 



R. fullonica, Linn., the " Sandback Ray" of the Brixham trawlers. In the 

 single specimen examined the canals of the posterior group of the hyoid ampullae 

 on the dorsal surface were five in number. They were of varying lengths, and th? 

 longest diverged from one another towards their extremities (Plate III, fig. 3). 



The dorsal outer canals of the hyoid ampullae were considerably more numerous 

 than in other " rays," numbering about fifty-five, as against from thirty to forty, but 

 there was nothing exceptional in the length of the last. two canals of the group. 



On the ventral surface (Plate III, fig. 4) the outer group of canals from the 

 outer buccal ampullae was less extended outwards over the pectoral fin than 

 in other species, most of the canals being directed backwards along the pro- 

 pterygium and lying close together, for the first part of their course at least in a 

 compact fascicle. 



R. radiata, Donovan. — Specimens of this species (Plate IV, figs. 5 and 6) were 

 examined in which the canals of the same posterior dorsal group numbered respec- 

 tively four and five, and in another specimen there were six on the left side and five on 

 the right. The specimen figured shows these canals of approximately equal length, 

 all except two diverging from one another towards their openings. 



R. circularis, Couch, known in Edinburgh as the "Medallion Skate," and in 

 Devonshire as the " Cuckoo Bay," has four or five canals in the dorsal posterior hyoid 

 group, of which one or two are shorter than the rest, while the longer ones are 

 divergent (Plate V, fig. 7). In the specimen figured it will be noticed that the most 

 posterior canal of the dorsal outer hyoid group is slightly shorter than those next to 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 12). 70 



