Page:American Seashells (1954).djvu/97

Rh Beyond each lateral there is first an inner marginal and finally an outer marginal. This makes seven teeth in all. In the rachiglossate snails (Muricidae, Buccinidae, Olividae, etc.) there are only three teeth per row—the rachidian and a strongly cusped lateral on each side. The four toxoglossate families (Conidae, Turridae, Terebridae and Cancellariidae) have lost their rachidians and laterals and have retained only the marginals.

The docoglossate snails (Acmaeidae and Patellidae) have less than twelve teeth per row but are peculiar in that there are two to four identical rachidians or centrals. In the rhipidoglossate families (Trochidae, Fissurellidae, Neritidae) the radula is very complicated, and the very numerous laterals at the end of each row are called uncini. Among the gastropods which do not have a radula are the Pyramidellidae, Eulimidae, the genus Coralliophila, adult Harpa and a few genera of nudibranchs.

We have figured several main types of gastropod radulae (fig. 25), but other examples have been included in the systematic section when they are of especial use in identification. It is not expected that many amateurs will want to prepare and examine radulae but, because so many serious private collectors and many biology students will find this identification tool indispensable, we have included brief instructions on the preparation of radula slides.

Preparation of the radula. In large specimens, such as the whelks or conchs, the proboscis may be slit open from above and the round buccal mass removed. Occasionally, the proboscis is withdrawn far inside the animal, but it is easily located below the thin skin on the dorsum just posterior to the tentacles. The flesh may be torn away with the aid of small dissecting needles until the glistening, worm-like radula pops out. In order to remove