This is one of the largest and most interesting families of the /Mollusca/. It contains many apparently widely separated genera and a host of species, which for the most part are littoral, the majority actually living between tide-marks. The typical trochid shell is top-shaped or pyramidal, having a broad base and many closely wound flat whorls terminating in a sharp apex. All the trochids are nacreous within the aperture—a character which is constant throughout the family. The animal has but one gill (the left), a short snout, and often frontal lobes on the head. The edge of the mantle or the epipodial line of the foot is usually ornamented with from three to five cirri. The tentacles are [pg360] long and slender, with short peduncles for the eyes; the operculum is corneous, with a central nucleus. The animals are herbivorous, feeding upon algæ. The trochids are essentially tropical shells, and the most of the genera which are comprised in the family are only to be encountered in the warm waters of the Pacific and Indian oceans. Some forms are very beautiful, and frequently are used as mantel ornaments, and the shells of one little species, which is opalescent in its coloring, are still extensively gathered in the East Indies, to be polished and strung like pearls in necklaces. GENUS ^Margarita^ On the Atlantic coast north of Cape Cod the trochids are represented by the genus /Margarita/, with five or six species. The shells are small, thin, and globosely depressed, with smooth or transversely striated whorls. The aperture is nearly circular, with a simple lip. [Illustration: /Margarita cinerea./] ^M. cinerea.^ This species has several prominent revolving ridges upon the upper side of the whorls, with finer ones on the base. Very fine growth-lines cover the entire shell. It ranges all along the coast north of Cape Cod, but is not usually found between tides. The writer has dredged many specimens in shallow water at Eastport and Bar Harbor. [Illustration: /Margarita helicina./] ^M. helicina.^ A thinner and more globose species than the last, with a translucent, shining, smooth surface of a yellowish or olive color. /M. helicina/ is very fond of the leaves of /Laminaria/, and is often found clinging to them when storms have torn these great algæ from the bottom and cast them upon the shore. At Bar Harbor they are common upon the eel-grass in Rodicks Weir. This species can generally be distinguished by its iridescent, metallic luster. [Illustration: /Margarita undulata./]