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346 DESTAL AXATO:iIY.

ing a general similarity to one another on either side of the rhachidian
row. In the mollusks where the sexes are divided (Dioeca) we find
single otoliths in the auditory sac, and the radula tending to a more
long and narrow form, with the lateral teeth, in general, less numerous
and showing much more diversity of form among themselves. To these
generalizations there are a few exceptions, as in most laws of wide appli-
cation, but which may be accounted for on other grounds. The former
type of dentition has been termed " pavemental," as recalling the uni-
form blocks of a granite pavement, and the other " ribbon-" or " strap-
like."
The highest type of dentition is that which has been called toxoglos-
sate (or arrow-toothed), and which consists of two longitudinal rows of
slender hollow or grooved teeth, each row set on a slender, flexible chit-
inous thread, apparently representing the pleurte of the odontophore.
Each tooth is usually provided with a duct, which conveys a poisonous
fluid to near the point, and the latter is frequently barbed or arrow-
shaped, from which the name is derived. Examples of this group are
Conus and Bela, both marine forms, the former tropical, the latter
Fig. 166.
Fig. 165.

Teeth of Bela. Teeth of Convs, showing barbs and poison-duct.
northern, in distribution. The animal of Conns milieus of the Moluc-
cas can give a severe bite. Admiral Sir Edward Belcher of the British
navv was bitten by one of them as he picked it out of the water, and
compared the acute pain which followed to the burning of phosphorus
under the skin. The bite, which was soon followed by a kind of blister,
Avas small, triangular, and deep. Troschel ^ has described the apparatus
of the gland and duct.
The Toxoghssa have no rhachis or rhachidian tooth, and no jaws ; in
some of them the series is reduced to a single pair of teeth, and for a
time these were supposed to be edentulous.
Next to the.se come the Rhachiglossa, of which the typical forms
( Voluta) have only a rhachidian tooth, but the larger number, such as
Fig. 167.

Fig. 169.

Rhachiglossate Teeth: 2 Fig. \C,7. .Single ihachidian tooth of rVw/a.—Fiir. K.R. Transverse series in
Ci/iwdu)ita.— Fig. 109. Transverse series in Fusus antiqui'is.
^ Gfh. rier Schneckrn. ii., 1S66. p. 15 rf .•'rq.
'^ All the figures of teeth show single transverse rows, unless otherwise stated.