“The student must be warned that though the general process is simple, there are difficulties in particular cases. In the Pulmonata, for example, membranes on both sides of the radula need careful removal. Murex, Purpura, and most of the Taenioglossa have the side teeth folded down over the central, so that the arrangement is not well seen till they have been brushed back. The Cones, again, have no basal membrane at all, so that if the potash is not used with great care, the single teeth will fall asunder and be lost. Perhaps the worst case is where a large animal has a radula as small as that of a Rissoa, like Turritella, Harpa, or Struthiolaria, or where the radula is almost filmy in its transparency, like those of Actaeon and the small Scalaria.
“When once the radula is laid out, the mounting is commonly easy. Canada balsam makes it too transparent. Fluids may be used, and are almost necessary for thick radulae like those of large Chitons; but the best general medium is glycerine jelly. It runs under the cover glass by capillary attraction, and may be boiled (though only for a moment) to get rid of air bubbles. It should then be left unfinished for several weeks. If cracks appear, the reason is either that the jelly is a bad sample, or that it has been boiled too long, or (commonly) that the object is too thick; and there is not often any difficulty in remounting. I have no serious complaint of want of permanence against the medium, if I may speak from a pretty wide experience during the last twenty years.”
[323] The substance both of the jaw and radula is neither crystalline nor cellular, but laminated. Chitin is the substance which forms the ligament in bivalves, the ‘pen’ in certain Cephalopoda, and the operculum in many univalves. Neither silica nor keratine enter into the composition of the radula.
[324] τόξον, arrow; ῥάχις, ridge, sharp edge; ταινία, ribbon; πτηνός, winged; γυμνὀς, bare; ῥιπίς, fan; δοκός, beam.
[325] V. concinna, according to Schacko (Conch. Mitth. i. p. 126, Pl. xxiv. f. 5); the lateral is large, strong, unicuspid on a broad base.
[326] In some cases (e.g. Hyalinia inornata) the laterals are very few, while in Zonites laevigatus the first side tooth is more of a marginal than a lateral.
[327] Semon, Biol. Centralbl. ix. p. 80.
[328] According to Moquin-Tandon (Moll. de France, i. p. 44) this process in Bithynia is attached by one end to the wall of the stomach. Vivipara, with two jaw pieces, does not possess this stylet; Bithynia, which does possess it, has no jaw.
[329] J. H. Vanstone, Journ. Linn. Soc. xxiv. p. 369.
[330] Biol. Centralbl. vii. p. 683; SB. Ges. Nat. Fr. 1890, p. 42; Mag. Nat. Hist. (2) v. 1850, p. 14.