IX 



VARIOUS FORMS OF THE SPIRAL 



247 



Sometimes, however, the coil of the whorls, instead of being 

 oblique, tends to become horizontal to the axis, and thus we 

 have another series of gradations of form, from the excessively 

 produced spire of Terehra to the flattened disc of Planorhis^ 

 Polygyratia^ Euomphalus^ and Ammonites. The shell of many 

 species of Conus practically belongs to the latter type, each whorl 

 folding so closely over its predecessor that the spiral nature of 

 the shell is not perceived until it is looked at at right angles to 

 the spire. 



In some cases the regularly spiral form is kept, but the 

 whorls are completely disconnected; e.g. some Scalaria^ Spirida; 



hi^a 



Fig. 151. — Examples of sheUs 

 with disconnected whorls; 

 A, Cyathopoma cornu Mf., 

 Philippines; B, Cylindrella 

 hystrix Wright, Cuba. (Both 

 x4.) 



Fig. 152. — Example of a shell A/rylJL^ 

 whose apical whorls alone are 

 coiled, and the remainder pro- / # 1^ 



duced in a regular curve. r>^\A^ *^ 

 {Cyclosurus Mariei Morel. 

 Mayotte.) 



\AA^ 



among fossil Cephalopoda, G-yroceras., Crioceras., ?ind Ancyloceras ; 

 and, among recent land Mollusca, Cylindrella hystrix and Cyatho- 

 poma 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 

 coiledin 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 irregu- 

 larly backwards, and is brought up to the apex, so that the animal 

 in crawling must carry the shell with the spire downwards, as in 



