external agency on the habit of Halimeda forms; it is rare. to find any record even of the 

 depth at which a specimen was found living. At the coral island of Funafuti in the S. Pacific, 

 plants of Halimeda grow at a depth of 45 fathoms on the outer slope of the reef, the principal 

 >ne being somewhere between 15 and 45 fathoms. The collections brought home from this 

 island are not however large enough to furnish data for any generalisation on the effect ot 

 depth or external form. 



The individual joints vary largely in size and form. They may be flat or rounded, and 

 either semicircular, cuneate, trilobed, or cylindrical; with or without a thickened margin; ribbed 

 or plane. Several varieties of joint may occur in one and the same plant. 



In many species longitudinal ridges, which mark the position of the filaments of the 

 central strand, may be seen running up the middle of each joint (internode). This is the ribbed 

 condition mentioned above. 



Cad [fication. As mentioned above, investigations on the calcification of Halimeda were 

 carried out by Professor Askenasy and as I have no new facts to add to his results I quote 

 his remarks in full. 



" Most plants of Halimeda secrete carbonate of lime within their thallus, thereby giving 

 rigidity and hardness to the joints. In rare cases only, the calcification is slight or entirelv 

 absent, as in the specimens of H. macroloba l ~) brought home by the Gazelle, in the oldest 

 joints of which calcification occurs to only a slight extent. As a rule the calcium carbonate is 

 deposited very early in certain parts, soon after the complete formation of the joint, and these 

 deposits increase with age. Certain parts remain however uncalcified, such as the outer surface 

 of the peripheral cells and in most cases the adjacent portion of their side walls. Hence it is 

 possible, even in the oldest joints, to remove this superficial layer in the form of a thin mem- 

 brane of uncalcified cellulose, by means of a section parallel to the surface. The only exception 

 is H macrophysa, in which the side walls of the peripheral cells are completely calcified, 

 while the outer surface remains soft and uncalcified. Consequently the peripheral cells do not 

 adhere together after treatment with acid, as is the case in other species, but lie quite free 

 near each other. The secretion of calcium carbonate begins on the outer surface of the side 

 walls of the peripheral cells and soon spreads over the entire space between them. The granules 

 appear first as very small dark spots, which from their number give at first, by transmitted 

 light, a brownish tint to those jaarts in which they occur. As the secretion continues, there is 

 gradually formed, between the peripheral cells, a firmly connected calcified plate, which soon 

 becomes quite opaque. A surface view of the thallus from above shews first the peripheral 

 network mentioned above and, at a lower focus, the dark calcified mass which is seen to be 

 interspersed with small, circular openings. These openings are the places where the stalks of 

 the peripheral cells pass through the layer of calcification (Kalkplatte). The same thing is seen 

 in a transverse section of the calcified peripheral layer. 



According to an analysis of Payen, published in Flora I. 1S44. p. 71 the substance of 

 Halimeda Opuntia consists of 90.16% calcium carbonate, 5.50 °/ magnesium carbonate, 0.54 °/ 

 calcium sulphate and silica and only 3.8 "/,, organic matter. Thin sections taken through the 



ij //. cuneata Her. 



