In some cases the regularly spiral form is kept, but the whorls are completely disconnected; _e.g._ some _Scalaria_, _Spirula_; among fossil Cephalopoda, _Gyroceras_, _Crioceras_, and _Ancyloceras_; and, among recent land Mollusca, _Cylindrella hystrix_ and _Cyathopoma cornu_ (Fig. 151). Sometimes only the last whorl becomes disconnected from the others, as in _Rhiostoma_ (see Fig. 180, p. 266), _Teinostoma_, and in the fossil _Ophidioceras_ and _Macroscaphites_. Sometimes, again, not more than one or two whorls at the apex are spirally coiled, and the rest of the shell is simply produced or coiled in an exceedingly irregular manner, _e.g._ _Cyclosurus_, _Lituites_, _Orygoceras_, _Siliquaria_ (Fig. 153), _Vermetus_. In _Coecum_ (Fig. 170, p. 260) the spiral part is entirely lost, and the shell becomes simply a cylinder. In a few cases the last whorl is coiled irregularly backwards, and is brought up to the apex, so that the animal in crawling must carry the shell with the spire downwards, as in _Anostoma_ (Fig. 154), _Opisthostoma_ (Fig. 208, p. 309), _Strophostoma_, and _Hypselostoma_ (Fig. 202 A, p. 302). [Illustration: FIG. 153.--_Siliquaria anguina_ Lam., showing scalariform coil of upper whorls and irregular extension of the lower.] [Illustration: FIG. 154.--_Anostoma globulosum_ Lam., Brazil. (After P. Fischer.)] [Illustration: FIG. 155.--Various forms of the internal plate in _Capulidae_: =A=, _Calyptraea_ (_Mitrularia_) _equestris_ Lam., E. Indies; =B=, _Crucibulum scutellatum_ Gray, Panama; =C=, _Ergaea plana_ Ad., and Reeve, Japan; =D=, _Galerus chinensis_ L., Britain; =E=, _Crepipatella dilatata_ Lam., Callao; =F=, _Trochita maculata_ Quoy, N. Zealand; =G=, _Crepidula fornicata_ Lam., N. America.] Some genera of the _Capulidae_, in which the shell is of a broadly conical form or with scarcely any spire, develop an internal plate or process which serves the purpose of keeping the animal within the shell, and does the work of a strong attachment muscle. In _Mitrularia_ this process takes the form of a raised horse-shoe; in _Crucibulum_ it is cup-shaped, with the edge free all round; in _Galerus_, _Ergaea_, _Crepipatella_, and _Trochita_ we get a series of changes, in which the edge of the cup adheres to the interior of the shell, and then gradually flattens into a plate. In _Crepidula_ proper this plate becomes a regular partition, covering a considerable portion of the interior (Fig. 155 G). _Hipponyx_ secretes a thin calcareous plate on the ventral surface of the foot, which intervenes like an operculum between the animal and the substance to which it adheres.