212 Puget Sound Marine Sta. Pub. Vol. 1, No. 19 



shape, Avith a round-lobed margin. It adheres tightly to the rock on 

 which it grows, sometimes attached only at a small central area, the lobes 

 being free. The more vigorous specimens are firm and have a smooth 

 velvety surface, but many plants are spongy and soft. The thallus rarely 

 exceeds 1 cm. in thickness and usually covers an area of about 20 sq. cm. 

 The lobes are often nearly separate from the rest of the plant, sometimes 

 attached to it by only a few filaments. This vegetative multiplication is 

 shown in figure 1 of plate 16 in Svedelius' (12) paper. 



The thallus is made up of pith filaments ending in utricles, which 

 are perpendicular to the surface and held tightly together by a gelatinous 

 substance so that they form a compact cortex. The individual pith-fila- 

 ments are expanded at their lower ends to form lobed holdfasts, which 

 are sometimes clear and sometimes densely green (Fig. 7). 



The pith-filaments are slender threads, almost colorless, 12 to 30/* 

 in diameter, rarely as wide as 4-5fx., with one or more utricles at the upper 

 end and a holdfast at the other (Fig. 6). One filament was measured 

 which was 10.3 mm. from the holdfast to the tip of the utricle, and which 

 had no branches or any visible connection with the rest of the plant. In 

 many respects the plant body of C. dimorphum resembles the holdfast 

 cushion of C. mucronatum. It has the same general shape, the same lobed 

 margin, the same internal structure. It seems probable that C. mucron- 

 atum has been derived from a form like C. dimorphum. If this be the 

 case, the erect fronds of the former are specialized structures for bearing 

 the reproducing utricles. 



The pith-filaments of C. dimorphum are usually without either cross- 

 walls or plugs. Very rarely thin partitions resembling cross-walls are 

 present in a filament; and one plug-like thickening was found, but its 

 structure was not clear and it did not look exactly like the true plugs of 

 C. mucronntum (Fig. 13). A plug is commonly present, however, in the 

 base of each gametangium and of each lateral branch of the utricle (Figs. 

 2-4). This absence of plugs in the filaments of a species which grows 

 horizontally appressed to the rocks, and hence has comparatively short 

 filaments, may indicate their function. It is obvious that the necessity 

 for cross-walls to hold the cell contents in place does not exist in these 

 filaments to the extent that it does in the long soft fronds of C. mucronatum. 



The mature utricles of C. dimorphum are 75-100/x in diameter. Oc- 

 casionally a very old one is considerably swollen a short distance from the 

 outer end, and its diameter at the widest part may be as much as 250;U, 

 (Fig. 2). The utricles are cylindrical, truncate, and slightly thick-walled 

 at the tip. They are more or less constricted about 225/i. from the end. 

 Usually they are somewhat enlarged just below this constriction so that 



