THE LATERAL SENSE ORGANS OF ELASMOBRANCHS. 479 



like the gelatinous tissue of the snout in transparency and want of colour that they 

 are not readily seen, and require more careful dissection and examination than do 

 the conspicuous canals of the hyoid ampullae. 



(4) Hyoid Canals. 



The canals of the hyoid ampullae opening on the dorsal surface are arranged in 

 four groups which, adopting the nomenclature of Sappey, may be distinguished as 

 anterior, outer, inner, and posterior (Plate I), (l) In the anterior group the canals 

 lie parallel to one another, and are directed forwards along the groove between the 

 anterior inner border of the pectoral fin and the cephalic region. They open close to 

 one another where this groove approaches the outer border of the disc. (2) The canals 

 of the outer group diverge outwards and fan-wise over the anterior and middle portion 

 of the pectoral fin, opening at short and approximately equal distances from its 

 outer border. The most posterior canals of the group are from three to four times 

 as long as those whose course is directed immediately outwards to the nearest 

 portion of the edge of the fin. (3) The inner group comprises a compact fascicle 

 of parallel or slightly divergent canals, directed inwards and backwards along the 

 anterior border of the branchial region, and ending in openings which form a semi- 

 circular curve near the median line between the two gill regions. The canals of this 

 group are of approximately equal length. (4) .The posterior group contains typically 

 fewer canals than any of the preceding groups. They are directed backwards in a 

 sinuous course along the junction line of the pectoral fin and the trunk. In some 

 species these canals present a succession of different lengths, the openings occurring 

 at intervals along the course of the group ; in other species they are all equally long. 

 The longest of them reach to near the posterior border of the fin, and these either 

 equal or exceed in length the longest of the canals of the outer group. Usually the 

 canals of this group are of larger calibre than others, and increase markedly in width 

 towards their openings ; for instance, in a specimen of Raia clavata measuring about 

 21 inches across, the width of these canals was 1'5 mm. near the ampullge, and 3 mm. 

 near the openings. 



The hyoid canals opening on the ventral surface, although showing a certain 

 correspondence in arrangement, are somewhat less definitely grouped than those of 

 the dorsal side (Plate II). They are best described as falling into two groups : an 

 inner and an outer, (l ) The inner group corresponds to the inner group on the dorsal 

 surface, and extends inwards and backwards along the upper and inner borders of 

 the gill region ; but instead of lying close together and having approximately equal 

 lengths, the canals in this case diverge to some extent, and, taking a more curving 

 course, end at varying distances between the ampullae and the extremity of the group. 

 (2) The outer group of canals (including Sappey's outer and posterior groups) may 

 be taken as corresponding to the anterior, outer, and posterior groups of the dorsal 

 surface, and like these collectively they spread out fanwise, extending forwards, out- 



