Fronds tubular, simple or branched, sometimes inflated. The tubes vary in size in different species and also in the same species, some being fine like a hair, others large, and flat or inflated. The genus is widely distributed and very abundant. Species of ^Enteromorpha^ grow on the bottoms of ships, and in nautical language are called /grass/. ^E. clathrata.^ Fronds thread-like, tubular, branched, and branches beset with numerous fine branchlets; densely tufted, soft. Common everywhere. (Plate IV.) ^E. compressa.^ Fronds long, slender, branched, tufted; branches simple, compressed, extending from main central branch, obtuse at ends, but attenuated at base. The species is very abundant everywhere, and is a useful plant for the aquarium. (Plate V.) ^E. intestinalis.^ Single, long, inflated tubes or sacs, obtuse at the apex, very attenuated at the base; fronds often crimped and twisted, resembling an intestine, whence the name. (Plate V.) [pg056] ^E. lanceolata.^ Formerly called /Ulva Linza/. Frond narrow, ribbon-like, six to twelve inches long, one inch to two inches wide; blunt or pointed at apex, tapering at base; attached by a disk; edges much ruffled; bright green, soft, thin. GENUS ^Monostroma^ This genus resembles /Ulva/, but is more delicate since it has but one layer of cells, as its name implies. The frond is usually sac-like at first, then breaks apart, leaving a thin, semi-transparent membrane of no definite shape. GROUP =SIPHONEÆ= The distinct and peculiar character of this group is that in each individual the whole plant consists of but one cell. There are many genera, some of which are plants of elaborate form and considerable size, but always the one cell expands and branches without dividing the elongated cavity with septa, or plates of division. In ^Caulerpa^ the stability of the plant is secured by numerous fibrils which emanate from the interior of the cell, forming a spongy network of interlacing filaments. In other orders the branches gain support from incrustation, from interlacing, and from cohering on the edges. ORDER =VALONIACEÆ= The algæ of this order are found only in tropical or subtropical waters. Their holdfasts resemble fibrous roots and penetrate the sand or coral on which they grow. GENUS ^Chamædoris^ ^C. annulata.^ When young this alga consists of an annulated tube formed of a single cell. The annular constrictions occur at short intervals, giving it the appearance of being jointed. It grows to the height of two to three inches, when it ceases to lengthen and produces a dense mass of filaments, forming a head or spherical tuft one inch or more in diameter. It is bright grass-green in color, rather rigid and tough, and when mature is thinly coated with carbonate of lime. The holdfast is a tuft of fibers. The species is found at Key West and is a native of the West Indies.