(2) (∞.1).4.1.4.(1.∞), regarding the ‘last lateral’ as first uncinus, but specialising it by a number. (3) ∞.1.(4.1.4).1.∞, regarding the ‘last lateral’ as the only lateral. In the Neritidae and the derived fresh-water genera (_Neritina_, _Navicella_) the first lateral, as well as the capituliform tooth, is very large, and in shape rather like the blade bone of a shoulder of mutton; the intervening laterals are very small. In _Neritopsis_ (a degraded form) the central tooth and first lateral are entirely wanting. In the neritiform land-shells (_Helicina_, _Proserpina_) the first lateral is no larger than the others, while the capituliform tooth is enormous. _Hydrocena_ is a very aberrant and apparently degraded form; the laterals between the first and the capituliform tooth are all wanting. In _Haliotis_, _Scissurella_, and _Pleurotomaria_ the five laterals are of fairly equal size; in _Fissurella_ we again meet with a large capituliform tooth, with very small laterals. (_g_) The _Docoglossa_ are in direct contrast with the Rhipidoglossa in possessing few and strong teeth, instead of many and weak. There are only three families, Acmaeidae, Patellidae, and Lepetidae. In some of the Acmaeidae there are not more than two teeth in a row, while in no species are there more than twelve. The radula is, however, very long; there are as many as 180 rows in _Patella vulgata_. The teeth are thick, generally of a very deep red horn colour, rather opaque. The cartilage in which they are set is remarkably thick, and in some species whose teeth are very few a considerable portion of this cartilage is left quite bare. [Illustration: FIG. 131.--Portion of the radula of _Patella cretacea_ Reeve, seen in half profile. × 40.] Although the teeth are so few, the arrangement is by no means simple. The special feature of the group is the multiplication of identical centrals. Of these there are two in _Acmaea_, and four, as a rule, in _Patella_. Thus in these two genera there is seldom an absolutely _central_ tooth. Either laterals or marginals are liable to be lost, but there are never more than two of either in _Acmaea_, and never more than two laterals and three marginals in _Patella_. Thus the formula varies from 0.0.(1 + 0 + 1).0.0 in _Pectinodonta_, 2.2.(1 + 0 + 1).2.2 in _Collisellina_ (both Acmaeidae), to 3.2.(1 + 0 + 1).2.3 in _Patinella_, and 3.1.(2 + 0 + 2).1.3 in _Patella_ proper. In the Lepetidae there is an absolutely central tooth, which appears to be made up of the coalescence of several teeth, no laterals, and about two marginals; formula, 2.0.1.0.2. [Illustration: FIG. 132.--Two rows of the radula of _Pterotrachea mutica_ Les., Naples. × 60.]