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 _Odostomia_, _Eulimella_, _Turbonilla_, 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 occasionally noticed among a crowd of individuals. More serious forms of monstrosity are those which occur in individual cases. Mr. S. P. Woodward once observed[332] a specimen of an adult _Helix aspersa_ with a second, half-grown individual fixed to its spire, and partly embedded in the suture of the body whorl. The younger snail had died during its first hibernation, as was shown by the epiphragm remaining in the aperture, and its neighbour, not being able to get free of the incubus, partially enveloped it in the course of its growth. In the British Museum two _Littorina littorea_ have become entangled in a somewhat similar way (Fig. 160 B), possibly as a result of embryonic fusion. Double apertures are not uncommon[333] in the more produced land-shells, such as _Cylindrella_ and _Clausilia_ (Fig. 160 A). In the Pickering collection was a _Helix hortensis_ which had crawled into a nutshell when young, and, growing too large to escape, had to carry about this decidedly extra shell to the end of its days. A monstrosity of the cornucopia form, in which the whorls are uncoiled almost throughout, is of exceedingly rare occurrence (Fig. 161). [Illustration: FIG. 158.--Monstrosities of _Neptunea antiqua_ L., and _Buccinum undatum_ L., with a greatly produced spire (from specimens in the Brit. Mus.).] [Illustration: Fig. 159.--Monstrosities of _Littorina rudis_ Mat, The Fleet, Weymouth. (After Sykes.)]