(d) Ptenoglossa.—This section consists of two families only, which certainly appear remarkably dissimilar in general habits and appearance, viz., the Ianthinidae and Scalariidae. In all probability their approximation is only provisional. The radula, which in Ianthina is very large, and in Scalaria very small, possesses an indefinite number of long hooked teeth, of which the outermost are the largest. The central tooth, if present (it does not occur in Ianthina), is the smallest in the series, and thus recalls the arrangement in some of the carnivorous Pulmonata (p. 232). In Ianthina the radula is formed of two large divisions, with a gap between them down the middle.
The formula is ∞.1.∞ or ∞.O.∞ according as the central tooth in Scalaria is or is not reckoned to exist.
(e) Gymnoglossa.—In the absence of both jaw and radula it is not easy to classify the two families (Eulimidae and Pyramidellidae) which are grouped under this section. Fischer regards them as modified Ptenoglossa; one would think it more natural to approximate them to the Taenioglossa.
Fig. 129.—Portion of the radula of Margarita umbilicalis Brod., Labrador. × 75 and 300.
(f) Rhipidoglossa.—This section consists of seventeen families, the most important being the Helicinidae, Neritidae, Turbinidae, Trochidae, Haliotidae, Pleurotomariidae, and Fissurellidae. The radula is characterised by—
(1) The extraordinary development of the uncini, of which there are so many that they are always reckoned as indefinitely numerous. They are long, narrow, hooked, and often cusped at the top, and crowded together like the ribs of a fan, those at the extreme edge not being set straight in the row, but curving away backwards as they become smaller; in Solariella alone, where there are from five to ten, can they be counted.
Fig. 130.—Portion of the radula of Nerita albicilla L., Andaman Is., with central tooth highly magnified: c, c, the capituliform tooth. × 40.
(2) The varying number of the laterals. The average number of these is five on each side; in some cases (Livona) there are as many as nine, in some (Neritopsis) only three. The lateral next to the uncini (which is specially large in the Neritidae, and is then known as the capituliform tooth) is regarded by some authorities as the first uncinus, by others as the sole representative of the laterals, the teeth on the inner side of it being reckoned as multiplied central teeth. According to this latter view, Livona will have as many as seventeen central teeth. Taking five as the average number of ‘laterals,’ we shall have the following different ways of constituting the rhipidoglossate formula, the first being that to which preference is given, viz.:—