Fig. 137.—Portions of the radula of Opisthobranchiata, illustrating types (b) and (c); A, Scaphander lignarius L.; A´, one of the teeth seen from the other side, × 40; B, Lamellidoris bilamellata L., Torbay, × 60; C, Hydatina physis L., E. Indies, × 75.
The first family of jaw-bearing snails, the Selenitidae, is distinctly intermediate. The possession of a jaw relates it to the main body of Helicidae, but the jaw is not strong, while the teeth are still, with the exception of the central, thoroughly Testacellidan. The central tooth is quite rudimentary, but it is something more than a mere weak reproduction of the marginals. There are no true laterals. The Limacidae show a further stage in the transition. Here the central tooth has a definite shape of its own, tricuspid on a broad base, which is more or less repeated in the first laterals; these, as they approach the marginals, gradually change in form, until the outer marginals are again thoroughly Testacellidan.[326] This is the general form of radula, varied more or less in different genera, which occurs in Nanina, Helicarion, Limax, Parmacella, and all the sub-genera of Zonites. It is certain that some, and probable that all of these genera will, on occasion, eat flesh, although their usual food appears to be vegetable. The jaw is more powerful than in the Selenitidae, but never so large or so strongly ribbed as in Helix proper.
Fig. 138.—Portion of the radula of Glandina truncata Gmel. × 40.
Fig. 139.—Portion of the radula of Rhytida Kraussii Pfr., S. Africa. × 25.
When we reach the Helicidae, we arrive at a type of radula in which the aculeate form of tooth—so characteristic of the Agnatha—disappears even in the marginals, and is replaced by teeth with a more or less quadrate base; the laterals, which are always present, are intermediate in form between the central and the marginals, and insensibly pass into the latter. In size and number of cusps the first few laterals resemble the central tooth; in the extreme marginals the cusps often become irregular or evanescent. As a rule, the teeth are set squarely in the rows, with the exception of the extreme marginals, which tend to slope away on either side. In some Helicidae there is a slight approximation to the Zonitidae in the elongation of the first marginals.
The above is the type of radula occurring in the great family Helicidae, which includes not only Helix proper, with several thousand species, but also Arion, Bulimus, Ariolimax, and other genera. The jaw is almost always strongly transversely ribbed.
In the Orthalicidae (Fig. [140], C) the teeth of the radula, instead of being in straight rows, slope back at an angle of about 45 degrees from the central tooth. The central and laterals are very similar, with an obtuse cusp on rather a long stem; the marginals become bicuspid.