No. 10.] FRESH- WATER ALCE. 15 



FAMILY I. RIVULARIACE.E. 



Abundant in mountainous regions, found principally on 

 dripping- rocks, in streams and waterfalls, or on the shores of 

 rocky lakes. Our collections not having included such locali- 

 ties, this family is not represented in our figures. The fila- 

 ments are all attenuated from a long base to a hair-like end. 

 One or two heterocysts are usually located at the base. The 

 sheath is yellow or yellowish-brown, gelatinous, tubular, and 

 often thoroughly lamellated. Asexual reproduction by hormo- 

 gones, and in Gloeotrichia and Calothrix asexual spores arise 

 near the basal heterocysts. 



Key to Genera. 



1. Without heterocysts Amphithrix* 



With heterocysts 2 



2. Filaments without gelatinous integument, simple, 



growing in branched or unbranched tufts, or some- 

 times singly , Calothrix* 



Filaments without gelatinous integument, branched, 

 several branches in a common sheath Dichothrix* 



Filaments with a gelatinous integument, forming a 

 gelatinous or mucous covering +. . . 3 



3. Filaments radially disposed ; thallus spherical or 



hemispherical in shape 4 



Filaments not radiating; thallus plain, cushion- 

 shaped Isactis 



4. Spores present; single-celled or with heterocysts; 



colonies free, floating Glaeotriclvia* 



Spores lacking ; colonies attached Rivularia* 



Description of Genera. 



Amphithrix Kiitz. The filaments form a thin expanded 

 stratum of a purple or violet color, which consists of two 

 layers. The inferior layer is composed of densely intricate 

 filaments, or of minute radiately disposed series of cells ; the 

 superior layer of simple erect filaments closely packed and at- 

 tenuated. 



[A. janthina (Mont.) Born, and Flah.] 



