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THE TEETH OF INVERTEBRATES. 347

the common whelk {Buccinum ; Fums, and Cijnodonta are also examples)
have one lateral tooth on each side of the rhachidian. These teeth are
straight, are usnally prettily dentioidated on the entting edge, and the
radula is long and strap-like. The jaw is represented by two lateral
rndiments, as in the next group. There are a few exceptional cases
where the tooth on the rhachis is reduced to an edentulous flat ])late.
The bases of the teeth point forward in this group.
The Tcenioglossa (bent-toothed) are a very extensive assembly, which,
amongst others, contains the largest part of our gill-breathing fresli-
FiG. 170.
Fig. 171.

Tfenioglossate Teeth. Fig. 170. Teeth o{ Rissoa—Fig. 171. Teeth of Vivipara.
water snails. The teeth are bent so that their cutting edges turn toward
the base of the tooth, which is con.se(|uently set on the odonto})hore with
the base as well as the point turning backward, as otherwise the crea-
tures would bite out of, instead of into, their own throats. They have
a rhachidian and three lateral teeth on each side of it, and in a few
cases a few uncini, but these are very exceptional. Rlssoa and Vivipara
are good examples of this sort of dentition.
A small group which has been called Ptenoglossa (feather-toothed)
is generally supposed to lie between the Rhachi- and Tcenioglossa. Sca-
Fig. 172.
\^AM
"j^^
Ptenoglossate Teeth : teeth of Scalaria.
laria, or the wentle-trap, is an example of this kind. The animals are
marine, carnivorous, and have numerous slender similar lateral teeth
with a bare rhachis and no uncini.
The Rhiphidoglossa (needle-toothed) comprise an immense variety of
marine snails and a few operculated land- and fresh-water snails, such
as Helicina, Neritina, Gibhula and Haliotis. The name is derived from

Rhiphidoglossate Teeth ; Teeth of Gibbula.
the immense number of needle-like uncini which exLst in many of the
species. The rhachidian is usually present ; the number of laterals is
variable ; the uncini always numerous and similar. They are set on the