936 PRINCIPLES OF STRATIGRAPHY 



The Phcuophycecc, or brown alg?e, are distinguished by the pos- 

 session of a brown coloring matter in addition to the chlorophyll. 

 The Peridiniacecc and Diatomacecc are included here, together with 

 the Cryptomonadacecc, all of them unicellular plants with little ex- 

 cept color in common with the true brown algae {Euphceophycece) , 

 which are multicellular. Familiar examples of the last class are 

 Fucus, Laminaria, and Sargassum. The kelps (Laininaria) de- 

 velop large round "stems" which branch root-like at the base and 

 have an oar-like expansion at the top. The rock-weeds (Fucus) de- 

 velop air-bladders which serve for purposes of flotation. They 

 are attached to the rock by means of a disc or root-like expansion ; 

 have a stem of rough leathery texture which forks regularly ; and 

 are expanded in a leaf-like manner with thick mid-ribs. The Gulf- 

 weeds (Sargassum) have distinct stems, leaves, and stalked air- 

 bladders, and strikingly resemble land plants. 



The diatoms are microscopic unicellular plants of a yellow or 

 reddish-brown color, and not closely related to the other algae except 

 perhaps to the desmids. The cell wall is impregnated with silica, 

 so that its shape is preserved after the death of the plant. The 

 "shell" consists of two parts, one overlapping the other like a pill- 

 box and cover. These show great variety of form and have the 

 power of locomotion. 



Of the true brown algae there are only about 620 species. The 

 Peridiniacecc and Cryptomonadacecc comprise only about 220 spe- 

 cies, but the diatoms, recent and fossil, include about 5,000 species. 



The Rhodophycece, or red algje, also called Floridecc, are so 

 named from the presence of a red color besides the chlorophyll. 

 Species growing near high-water mark are generally of a dark hue 

 and may be mistaken for brown algae. The Irish moss, Chondrus, 

 is a good example. Those growing near low-water, or in the shade 

 of other algae, are bright colored. They are all multicellular and 

 mostly microscopic in size, but some large species occur. Lime- 

 secreting forms are common, the branching Corallina, the encrust- 

 ing JMelobesia and Lithothamnion abounding both in recent and 

 fossil state. The total number of species of red algae is about 1,400; 

 this, together with the brown algae, 840 species, the diatoms, 5,000 

 species, and the 3,000 species of green algae, makes a total of over 

 10,200 species of algae. 



Fucoids. This is a general term applied to impressions on 

 rocks, suposed to represent sea-weeds. In some cases land plants 

 and even traces of inorganic structures have been included here. 

 Ex. : Fucoides verticalis of the Portage, probably a land plant ; 



