^B. occidentalis.^ This species, which is found in the waters of Florida on all sandy beaches, is a relative of the Northern species just described. The shell is larger than that of /Haminea solitaria/, and has a color-pattern of mottled or clouded reddish-brown on a white foundation. There is no spire; there is a pit in place of an apex; and the aperture extends the full length of the shell. To the naked eye the surface appears smooth and shining. It is an exceedingly variable species, occurring all through the West Indies, but not north of Florida. The shores of the keys along the west coast of Florida are often strewn with these shells after storms. The animal, like all the shell-bearing opisthobranchs, is very large as compared with the shell, and the large mantle folds recurve upward, almost completely hiding the shell from view. (Plate LXV.) ^B. nebulosa.^ This is one of the largest and finest species of the family /Bullidæ/. It is found on the coast of California south of San Francisco. The shell is much larger than that of either of the Atlantic species mentioned above, but very closely resembles them in outline, although it is built upon a more generous plan and is more richly painted. It is brownish, mottled with white and yellow patches, and is very suggestive of certain kinds of birds' eggs. The animals have a greatly extended mantle which almost completely envelops the shell. The foot is extremely large, with great wing-like developments upon each side, called parapodia. Some species of /Bulla/ have been seen to swim by means of the lazy flapping of the parapodia. (Plate LXV.) SEA-HARES Closely allied to these outer-shell-bearing opisthobranchs is a large and important group of tectibranchs, known as [pg351] "sea-hares." They have a mantle so greatly developed that it actually covers the shell, and its edges unite and fuse over the top. The shell, in consequence, having almost ceased to be of use as a protection, has degenerated into a mere horny plate, and has lost all resemblance to the ordinary gasteropod shell. Having practically lost its protective office and become a mere internal plate, it is quite probable that it will in time wholly disappear. The gills of the sea-hares are concealed under a flap of the mantle, their position being posterior to the heart. The most conspicuous representative of this type of tectibranchs in the United States is the following: GENUS ^Aplysia^