$2 THE GASTROPODA 
monstrosity has been sometimes fixed by heredity), Helix, Avion, 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- 
JURODOIIRY 
Fic. 64. 
Passage from a sinistral orthostrophic form (a) to a pseudo-dextral hyperstrophic one (6) ; 
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 which 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. Vermetus, Turritella, Caecum (Fig. 68), 
Truncatella, Triton (Fig. 66), Cuviertna, etc. In the families 
Cylindrellidae, Stenogyridae (Lwimina decollata), and Pupidae, and 
