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]).
Fig. 158.—Monstrosities of Neptunea antiqua L., and Buccinum undatum L., with a greatly produced spire (from specimens in the Brit. Mus.).
Fig. 159.—Monstrosities of Littorina rudis Mat, The Fleet, Weymouth. (After Sykes.)
Some decades ago ingenious Frenchmen amused themselves by creating artificial monstrosities. H. aspersa was taken from its shell, by carefully breaking it away, and then introduced into another shell of similar size (H. nemoralis, vermiculata, or pisana). At the end of several days attachment to the columella took place, and then growth began, the new shell becoming soldered to the old, and the spiral part of the animal being protected by a thin calcareous envelope. A growth of from one to two whorls took place under these conditions. The individuals so treated were always sordid and lethargic, but they bred, and naturally produced a normal aspersa offspring.[334] In the British Museum there is a specimen of one of these artificial unions of a Helix with the shell of a Limnaea stagnalis.
