Fig. 151.—Examples of shells with disconnected whorls; A, Cyathopoma cornu Mf., Philippines; B, Cylindrella hystrix Wright, Cuba. (Both × 4.)
Fig. 152.—Example of a shell whose apical whorls alone are coiled, and the remainder produced in a regular curve. (Cyclosurus Mariei Morel., Mayotte.)
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).
Fig. 153.—Siliquaria anguina Lam., showing scalariform coil of upper whorls and irregular extension of the lower.
Fig. 154.—Anostoma globulosum Lam., Brazil. (After P. Fischer.)
