Page:Natural History, Mollusca.djvu/229

Rh œsophageal cerebral ganglions, and completeness of the circulation, it has established its claims as a Gasteropod. There are also traces of alliance with some of the inferior classes. The red blood and vermiform configuration of the posterior part of the animal show some of the characters of the Annelida."

Two species of the genus are recognised as British, D. entalis, the Smooth Tusk, and D. Tarentinum, the Grooved Tusk. Of these the former is common around the shores of Scotland and the north of England; the latter on our southern coasts. Though possessing much resemblance to each other, they may be distinguished by the small end of the latter being finely grooved lengthwise, while the former is quite smooth and shining.

The Smooth Tusk, here represented, grows to a length of nearly two inches, with a diameter of about a quarter of an inch at the larger end. It is opaque, of a shining white surface, like porcelain, never tinged (as its southern congener usually is) with pink, and never marked with sculpture, but often with rings indicating the progress of growth.

It is a deep-water species, living "buried in sand