82 



THE GASTROPODA 



monstrosity has been sometimes fixed by heredity), Helix, Anon, and 

 various other Pulmonates. There are, however, forms in which the 

 coil is hyperstrophic : in this case the whorls which form the spire 

 are very slightly prominent ; the spire becomes flatter and flatter, 

 and finally becomes concave and is transformed into a false umbilicus. 

 At the same time the part corresponding to the umbilicus (the 

 cavity opposite to the spire) of normally coiled forms becomes 

 prominent and constitutes a false spire. The coil then appears to 

 be sinistral, although the asymmetry of the organisation remains 

 dextral, as, for example, in Lanistes and the coiled thecosomatous 

 Pteropods, in which the opercular spiral follows the same direction 

 as the apparent spire of the shell (Fig. 49) ; or reciprocally in 

 Planorbis, especially in individuals which are scalariform or terato- 

 logically unrolled, such as Choanomphalus and Pompholyx (Fig. 64). 

 Finally, it may sometimes be observed that the spiral in which the 

 coil is formed insensibly changes its nature or its apparent direction 

 after the first larval whorls are completed. This is the pheno- 



FIG. 04. 



Passage from a sinistral orthostrophic form (a) to a pseudo-dextral hyperstrophic one (b); 

 the heart is indicated in black, in order to show the constancy of the sinistral organisation. 

 (After J. W. Taylor.) 



menon of heterostrophy, in which the spiral from being negative 

 eventually becomes positive ; that is to say, the coil that was at 

 first hyperstrophic becomes finally orthostrophic. Examples of 

 this phenomenon are Solarium (the larval shell of which has been 

 called Agadina), Mathilda, the Pyramidellidae (Fig. 65), Melampus, 

 and various Bullidae. 



The line along Avhich two successive whorls of the shells cease 

 to be in contact with one another is the " suture." The portion of 

 the shell separating the successive whorls of the visceral spire may 

 be resorbed in certain cases (many Auriculidae, some Neritidae, 

 Cypraea, Olivella, etc.), resulting in the concrescence of the whorls 

 of the visceral sac, or even in the suppression of its coils, as may 

 be seen in several species of the genus Auricula (Fig. 67). On 

 the other hand, the animal may desert the first whorls of the coiled 

 shell, and cut itself from them by the formation of a transverse 

 partition or septum : this operation may, in certain cases, be 

 repeated several times, e.g. Fermetus, Turritella, Caecum (Fig. 68), 

 Truncatella, Triton (Fig. 66), Cuvierina, etc. In the families 

 Cylindrellidae, Stenogyridae (Rumina decottata), and Pupidae, and 



