250 ULTRA-DEXTRAL AND ULTRA-SINISTRAL SHELLS chap. 



Ihering, and Pelseneer. The shell, in all these cases, is not 

 really sinistral, but ultra-dextral. Imagine the whorls of a 

 dextral species capable of being flattened, as in a Planorhis^ and 

 continue the process, still pushing, as it were, the spire down- 

 wards until it occupies the place of the original umbilicus, 

 becoming turned completely ' inside out,' and we have the whole 

 explanation of these puzzling forms. The animal remains 

 dextral, the shell has become sinistral. A convincing proof of 

 the truth of this is furnished by the operculum. It is well 

 known that the twist of the operculum varies with that of the 

 shell ; when the shell is dextral, the operculum is sinistral, with 

 its nucleus near the columella, and vice versa. In these ultra- 

 dextral shells, however, where it is simply the method of the 

 enrolment of the spire that comes in question, and not the 

 formation of the whorls themselves, the operculum remains 

 sinistral on the apparently sinistral shell. 



The reverse case to this, when the shell is dextral but the 

 orifices sinistral, is instanced by the two fresh-water genera 

 Pompholyx (from N. America), and Choanomphalus (L. Baikal). 

 A similar transition in the enrolment of the whorls may be 

 confidently assumed to have taken place, and the shells are 

 styled ultra-sinistral. 



Yet another variation remains, in which the embryonic form 

 is sinistral, but the adult shell dextral, the former remaining 

 across the nucleus of the spire. This is the case with Odos- 

 tomia^ Eulimella^ TurboniUa^ and Mathilda, all belonging to the 

 Prosobranchiata, with Actaeon, Tornatina, and Actaeonina among 

 the Opisthobranchs, and Melampus alone among Pulmonates. 



Monstrosities of the Shell. — Abnormal growths of the 

 shell constantly occur, some of them being scarcely noticeable, 

 except by a practised eye, others of a more serious nature, 

 involving an entire change in the normal aspect of the creature. 

 Scalariform monstrosities are occasionally met with, especially 

 in Helix and Planorbis, when the whorls become unnaturally 

 elevated, and sometimes quite disjoined from one another ; 

 carinated monstrosities develop a keel on a whorl usually smooth ; 

 acuminated monstrosities have the spire produced to an extreme 

 length (Fig. 158) ; sinistral monstrosities (see above) have the 

 spire reversed : dwarfs and giants, as in our own race, are occa- 

 sionally noticed among a crowd of individuals. 



