The radula of the _Heteropoda_ is quite characteristic, and shows no sign of affinity with any other Prosobranchiate. The central tooth is large, broad, tricuspid, and denticulated on a broad base; the single lateral is strong, often bicuspid; the two marginals simple, long, falciform; formula, 2.1.1.1.2 (Fig. 132). [Illustration: FIG. 133.--=A=, Portion of the radula of _Chiton_ (_Acanthopleura_) _spiniger_] Sowb., Andamans, × 30; =B=, portion of the radula of _Dentalium entalis_ L., Clyde, × 50.] =Amphineura.=--(_a_) _Polyplacophora._--The radula of the _Chitonidae_ is quite unique. It resembles that of the _Docoglossa_ in being very long, and composed of thick and dark horn-coloured teeth. The number of teeth, however, is considerably greater, amounting almost invariably to seventeen in each row. There are three rather small central teeth, the two outer of these being similar; next comes a very large lateral (the _major_ lateral), usually tricuspid, which is followed by two much smaller laterals, which are scarcely more than accessory plates; then a very large and arched marginal (the _major_ uncinus), at the outer side of which are three accessory plates. Some consider there is only one central tooth, and count the two small teeth on each side of it as laterals. Thus the formula is either (3 + 1).(2 + 1).3.(1 + 2).(1 + 3) or (3 + 1).(2 + 1 + 1).1.(1 + 1 + 2).(1 + 3). (_b_) _Aplacophora._--Of this rather obscure order, _Chaetoderma_ has a single strong central tooth, _Neomenia_ has no radula, _Proneomenia_ and _Lepidomenia_ have about twenty falciform teeth, much larger at one end of the radula than the other; formula, 0.1.0. =Opisthobranchiata.=--The radula of the Opisthobranchiata is exceedingly variable in shape, size, and number and character of teeth. Not only do allied families differ greatly from one another, but allied genera often possess radulae widely distinct in plan. Thus, among the Polyceridae, _Goniodoris_ has no central tooth, one large lateral and one marginal (form. 1.1.0.1.1); _Doridunculus_ the same, with five marginals (form. 5.1.0.1.5); _Lamellidoris_ one each of median, laterals, and marginals (1.1.1.1.1); _Idalia_, _Ancula_, and _Thecacera_ the same as _Goniodoris_; _Crimora_ several each of laterals and marginals. Even species of the same genus may differ; thus the formula for _Aeolis papillosa_ is 0.1.0, but for _Ae. Landsbergi_ 1.1.1; for _Philine aperta_ 1.0.1, but for _Philine pruinosa_ 6.0.6. [Illustration: FIG. 134.--Two teeth from the radula of _Aeolis papillosa_ L. × 55.]