94 Vnivcrsitij of Calif ornia PnJ)li<xitions in Botany [Vol.8 



lariaceae on the Pacific Coast of North America will undoubtedly be 

 considerably increased as more thorough exploration and study is 

 carried on. 



Key to the Genera. 



1. Sheaths free, not diffluent 2 



1. Sheaths diffluent into an enclosing jelly 3 



2. Trichomes single in each sheath 25. Calothrix (p 94) 



2. Trichomes few in a common sheath 26. Dichothrix (p 103) 



3. Trichomes radiating in a globular or lobulated thallus 28. Rivularia (p 105) 



3. Trichomes parallel in a flattened thallus 27. Isactis (p 104) 



25. Calothrix Ag. 



Filaments simple or with false branching, and with cylindrical 

 sheaths, usually aggregated into dense pulvinate masses; trichomes 

 tapering from near the base upward or only at the upper end, fre- 

 quently terminating in long hyaline hairs; spores or resting cells 

 have been observed in some species; heterocj^sts basal or intercalary, 

 single or several in a series. 



Agardh, Syst. Alg., 1824, p. xxiv (diagnosis), 70 (list of species). 



The genus Calothrix was founded by C. A. Agardh in 1824 (p. xxiv 

 diag.) with C. confervicola (Roth) Ag. as the type. In his account 

 Agardh {loc. cit., p. 70) mentions twelve species, four of which still 

 remain as true species of Calothrix. The genus is comparatively dis- 

 tinct, although some species are destitute of heterocysts and some lack 

 a distinct terminal hair. Some species are branched and one species 

 is decumbent in the middle and erect at both ends. Resting spores 

 are known in some species. The species of Calothrix are the simplest 

 of the Rivulariaceae, passing into those of Dichothrix, and thence 

 through Polythrix to Rivularia. Species of Calothrix with undevel- 

 oped hairs bear a certain resemblance to species of Microchaete, which 

 is usually placed among the Scytonemataceae. 



The species of Calothrix are often troublesome to determine or to 

 distinguish from one another, and the material from our coast is not 

 so abundant as is desirable. We have attempted to place our species 

 as accurately as possible, but the whole genus needs careful revision. 



Key to the Species. 



1. Heterocysts basal 2 



1. Heterocysts basal and intercalary 7 



2. Filaments over 20y". thick, epiphytic 1. C. consociata (p 95) 



2. Filaments 20/u. thick or, more commonly, more slender 3 



3. Plants in more or less extended layers, saxicolous 4 



