Onchidiidae: Rows oblique at the centre, straight near the edges; central strong, tricuspid; laterals and marginals very long, falciform, arched, unicuspid.
Auriculidae: Teeth very small; central narrow, tricuspid on rather a broad base; laterals and marginals obscurely tricuspid on a base like Succinea.
Limnaeidae: Jaw composed of one upper and two lateral pieces; central and lateral teeth resembling those of Helicidae; marginals much pectinated and serriform (Fig. [141], A). In Ancylus proper the teeth are of a very different type, base narrow, head rather blunt, with no sharp cusps, teeth similar throughout, except that the marginals become somewhat pectinated (Fig. [141], B); another type more resembles Limnaea.
Fig. 141.—Portions of the radula of A, Limnaea stagnalis L., with the central tooth and two first laterals, and two of the marginals, very highly, magnified; B, Ancylus fluviatilis Müll., with two of the marginals very highly magnified; C, Physa fontinalis L., with central tooth and two of the marginals very highly magnified.
Physidae: Jaw simple, but with a fibrous growth at its upper edge, which may represent an accessory plate; radula with very oblique rows, central tooth denticulate, laterals and marginals serriform, comb-like, with a wing-like appendage at the superior outer edge (Fig. [141], C).
Chilinidae: Central tooth small, cusped on an excavated triangular base, marginals five-cusped, with a projection as in Physa, laterals comb-like, serrations not deep.
Amphibolidae: Central tooth five-cusped on a broad base, central cusp very large; two laterals only, the first very small, thorn-like, the second like the central tooth, but three-cusped; laterals simple, sabre-shaped.
Scaphopoda.—In the single family (Dentaliidae) the radula is large, and quite unlike that of any other group. The central tooth is a simple broad plate; the single lateral is strong, arched, and slightly cusped; the marginal a very large quadrangular plate, quite simple; formula, 1.1.1.1.1 (Fig. [133], B).
Cephalopoda.—The radula of the Cephalopoda presents no special feature of interest. Perhaps the most remarkable fact about it is its singular uniformity of structure throughout a large number of genera. It is always very small, as compared with the size of the animal, most of the work being done by the powerful jaws, while the digestive powers of the stomach are very considerable.