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. In the _Bulimulidae_, which include the important genera _Placostylus_, _Amphidromus_, _Partula_, _Amphibulimus_, and all the groups of South American _Bulimulus_, the jaw is very characteristic, being thin, arched, and denticulated at the edges, as if formed of numerous narrow folds overlapping one another. The radula is like that of the Helicidae, but the inner cusp of the laterals is usually lengthened and incurved. In _Partula_ the separation between laterals and marginals is very strongly marked. The remaining families of Pulmonata must be more briefly described. In the _Cylindrellidae_ there are three distinct types of radula: (_a_) Central tooth a narrow plate, laterals all very curiously incurved with a blunt cusp, no marginals (Fig. 140, D); (_b_) radula long and narrow, central tooth as in (_a_), two laterals, and about eight small marginals; (_c_) much more helicidan in type, central and laterals obtusely unicuspid, marginals quite helicidan. Type (_c_) is restricted to Central America, types (_a_) and (_b_) are West Indian. _Pupidae_: Radula long and narrow; teeth of the helicidan type, centrals and laterals tricuspid on a quadrate base, marginals very small, cusps irregular and evanescent. This type includes _Anostoma_, _Odontostomus_, _Buliminus_, _Vertigo_, _Strophia_, _Holospira_, _Clausilia_, and _Balea_. _Stenogyridae_, including _Achatina_, _Stenogyra_, and all its sub-genera: Central tooth small and narrow, laterals much larger, tricuspid, central cusp long, marginals similar, but smaller.