NQSTOCACEAE. 493 



Family NOSTOCACEAE. 



Nostoc commune Yauch. is not uncommon both on moist ground and on 

 ground that is apparently dry a good deal of the time. It forms a con- 

 spicuous olive-green or nearly black membranous crust that is gelatinous when 

 moist and rather brittle when dry. It often, especially when dry, appears to 

 lie loose on the ground, without attachments of any sort. The more or less 

 confluent thalli sometimes appear to be several inches broad and show ele- 

 vated lobes and bullae and very irregular pits and lacunae on the upper sur- 

 face. Under a compound microscope, the cells, imbedded in a gelatinous 

 matrix, look like chains of beads, with occasional yellowish usually larger cells 

 known as heterocysts. 



Family SCYTONEMATACEAE. 



Scytonema ocellatum (Dillw.) Thuret, forms a dark almost black turf of 

 minute intricate or suberect threads on the sand dunes of Paget. In the 

 Scytouemataceae the sheaths of the filaments are firm and are scarcely gelat- 

 inous even when wet ; the filaments often show a so-called ' ' false ' ' branching ; 

 and, as in most of the other genera of the family, there are heterocysts some- 

 what like those of Nostoc. 



Scytonema myochrous Ag., which, like the former, can hardly be con- 

 sidered a marine species, forms a short nap or felt on rocks, as about Harring- 

 ton Sound. 



Scytonema junipericola Farlow, forms dark velvety patches on the bark 

 of the Bermuda cedar. 



Family STIGONEMATACEAE. 



Hapalosiphon intricatus W. & G. S. West, a delicate fresh-water fila- 

 mentous species, has been reported by Collins from the Devonshire marshes, 

 where it occurs in ditches, with Sphagnum. In this genus the filaments show 

 "true" lateral branching and intercalary heterocysts are present. 



Family RIVULARIACEAE. 



Rivularia polyotis (Ag.) Born. & Flah. forms small blackish green 

 sinuose-bullate gelatinous cushions on rocks and other objects between the 

 tide lines. In the Rivulariaceae there is a distinct differentiation of base and 

 apex of the filament, the apex running out into a thin hair. In Eivularia, the 

 filaments have a more or less radial arrangement and there is a heterocyst at 

 the base of each filament. 



Calothrix scopulorum ("Web. & Mohr) Ag. has been found by Mr. 

 Collins at Shelly Bay, where it formed a blackish green layer on a rock near 

 the high-water mark. 



Polythrix corymbosa (Harv.) Grun. forms a turf on rocks just below the 

 low-water mark. The erect subdichotomously branched fastigiate blue-green 



