CLASS II. PTEROPODA.
This is a very small Class, comprising a few species of curious structure. They are all of diminutive size, and all swim in the open ocean, rarely approaching the shore, except when washed thither by accident. They are all characterised by having a membranous expansion, resembling a large fin, on each side of the head. By means of these organs, the little Pteropod rows itself about in the open sea perpetually; being unfurnished with any means of crawling, or of affixing itself to any solid body. Some of these animals, as the genera Hyalæa (a) and Cleodora (b), for example, have the body enclosed in a shell of elegant form, and of