A striking instance of the comparative valuelessness of the shell alone as a primary basis of classification is furnished by the large number of cases in which a _limpet-shaped_ shell is assumed by genera widely removed from one another in cardinal points of organisation. This form of shell occurs in the common limpet (Patellidae), in _Ancylus_ (Limnaeidae), _Hemitoma_ (Fissurellidae), _Cocculina_ (close to Trochidae), _Umbrella_ and _Siphonaria_ (Opisthobranchiata), while in many other cases the limpet form is nearly approached. Roughly speaking, about three-quarters of the known Mollusca, recent and fossil, possess a univalve, and about one-fifth a bivalve shell. In _Pholas_, and in some species of _Thracia_, there is a small accessory hinge plate; in the Polyplacophora, or Chitons, the shell consists of eight plates (see Fig. 2, p. 8), usually overlapping. A certain proportion of the Mollusca have no shell at all. In many of these cases the shell has been present in the larva, but is lost in the adult. The shell may be (1) _External_, as in the great majority of both univalves and bivalves. (2) _Partly external_, _partly internal_; _e.g._ _Homalonyx_, _Hemphillia_, some of the _Naticidae_, _Scutum_, _Acera_, _Aplustrum_ (Figs. 148 and 149). [Illustration: FIG. 148.--_Aplustrum aplustre_ L. Mauritius, showing the partly internal shell (=S=); =F=, foot; =LL=, cephalic lappets; =TT=, double set of tentacles. (After Quoy and Gaimard.)] [Illustration: FIG. 149.--_Sigaretus laevigatus_ Lam., showing shell partially immersed in the foot; =F=, anterior prolongation of the foot. (After Souleyet.)] (3) _Internal_; _e.g._ _Philine_, _Gastropteron_, _Pleurobranchus_, _Aplysia_, _Limax_, _Arion_, _Hyalimax_, _Parmacella_, _Lamellaria_, _Cryptochiton_, and, among bivalves, _Chlamydoconcha_. (4) _Absent_; _e.g._ all _Nudibranchiata_ and _Aplacophora_, many _Cephalopoda_, a few land Mollusca, _e.g._ all _Onchidiidae_, _Philomycus_, and _Vaginula_.