THE MARINE BOTANIST. 103 



varying in different localities, and changing to a 

 blacker hue in drying : the extreme tips of the 

 slender and entangled branches of this plant are 

 rolled inwards like a ram's horn. When viewed 

 through the microscope, the surface-cells of the 

 frond look like a delicate and regular piece of 

 mosaic-work. In Rytiphlsea, the frond is trans- 

 versely striate and reticulated, four British species 

 are included in this genus : R. pinastroides is 

 remarkable for its secund and usually hooked 

 pinnae. R. complanata, the only species with 

 flattened fronds, is of rare occurrence. R. thuyoides, 

 a much commoner plant, " may always be known 

 from it," says Dr. Harvey, '' by its darker colour, 

 cylindrical stems, and generally by a narrower 

 frond. In ramification and general habit there is 

 much similarity. The two may sometimes be 

 found growing in close proximity, and even mixed 

 together; but I have generally noticed that 

 R. thuyoides, which is the stiffest in substance, 

 usually grows in shallow parts of the tide-pool, 

 sometimes standing out of the water, while R. com- 



