114 
THE VOYAGE OF H.M.S. CHALLENGER. 
The removal of the mandibles lays bare the anterior portion of the muscular mass, which 
bears the impress of their form. In the upper half the internal margin forms a thinner 
wing of about 3 mm. in breadth. — The walls of the hinder part of the buccal cavity, 
and the tongue for the most part, are reddish-brown, inclining to black ; the tectum 
radulse is black. — The tongue (fig. 6) entirely fills the hinder part of the buccal cavity, 
and is of the usual form, with a deep median cleft ; the length of the tongue is about 
2'6 cm., its breadth 3'5 cm., and its height 2'6 cm.; the radula is glittering blackish- 
brown, yellower within the sheath. The radula is made up of fifty-four or fifty-five 
series of teeth ; further back there are sixteen fully developed and four not fully 
developed series ; the total number is thus seventy-five ; the radula and its continuation 
in the sheath have, when removed, an entire length of 4 '5 cm., the breadth of the hinder 
part, when expanded, being 4'5 cm. The twenty anterior series of teeth were incomplete, 
specially on the median and outermost parts (PI. XIII. figs. 23, 24); in the youngest 
series there were 128 to 130 on each side of the median tooth. The length of the 
youngest median plates is about ’5 mm. by a height of '25 mm.; the height of the first 
lateral plate was about '43 mm. ; the length of the basal portion of the lateral plates 
increasing to '8 mm. by '58 mm. in height; the length of the nine outermost teeth was 
•2, '22, '25, '28, '3, '35 mm. up to ’4 mm.; the height of the outermost plate about 
'35-'4 mm. The colour of the teeth is chitinous yellow, becoming darker at the basal por- 
tion ; the teeth within the sheath much lighter in colour than the others. The median 
tooth has an elongate quadrangular basal portion, the middle of the posterior margin being 
more (PL XIII. figs. 7-9; PI. XIV. fig. 6, a) or less (PI. XIV. fig. 7) prominent ; the anterior 
part of the tooth rises in an obliquely directed short strong hook, the posterior margin of 
which is somewhat excavated at the base. The first four or six lateral plates are rather 
broader than the rest, and have a shorter hook (PI. XIII. figs. 10-15) ; the first (PI. XIV. 
fig. 6, h) often has a median prominence (PI. XIII. fig. 76, 6), just as in the median teeth. 
The teeth then gradually assume the normal form, but often show remarkable irregulari- 
ties, especially in the breadth of the basal part (figs. 1G, 17); the fourth or fifth is 
remarkable in this respect (fig. 13). All the following lateral teeth are of the, same shape, 
with a rather narrow basal part (PL XII. fig. 19), and with the hook oblique, somewhat 
crooked pointed., and narrower in the back than the base (fig. 16). There are frequently 
irregularities, which sometimes take the form of a coalescence of two teeth (Pl. XIII. fig. 22) ; 
sometimes the hook is shorter than usual (fig. 16, a), or completely absent; occasionally 
(fig. 22) the back is deeply excavated. In the exterior sixth or seventh part of the series 
the size of the teeth gradually decreases (Pl. XII. fig. 19), mainly owing to the diminution 
in size of the hook, which in the outermost is quite rudimentary (Pl. XIV. figs. 9, 10). 
The salivary glands are whitish and somewhat flattened, as far as could be made out, 
and rather short; the salivary ducts are strong, and about 10-12 mm. long (Pl.XIII.fig. 2 ,d). 
The oesophagus (PL XIII. fig. 2, e) is short and wide, about 2 cm. long, the inside pro- 
