282 FRESH-WATER ALGJ3 OF THE UNITED STATES. 



b. Trichomes homogeneous, regular. 



1. Frond very small ; trichomes very com- 

 pact 14. MINUTISSIMUM. 



"2. P^rond attaining 2-10 mm. . . .15. GREGARIUM. 



3. Fronds attaining the size of a large 

 cherry 16. PRUNIFORME. 



VI. VERRUCOSA. Aquatic species ; fronds rounded 

 or discoid, at first full, then hollow ; periderm firm 

 and tough ; trichomes thin, regular. 



a. Fronds subglobose or warty; mucilage 

 tolerably soft; trichomes medium, compact; 

 sheaths not frequent. 17. VERRUCOSUM. 



b. Fronds at first discoid, or tongue-shaped, 



a little ovoid, hard 18. ALPINUM. 



VII. ZETTERSTEDTIANA. Aquatic species. Fronds 

 spherical, hard, tuberculose, divided into radiating 

 separable lobes. 19. ZETTERSTEDTII. 



VIII. FLAGELLIFORMIA. Terrestrial species ; fronds 

 forming thongs, linear, straight and long; subdicho- 

 tomous; trichomes longitudinally parallel. . . 20. FLAGELLIFORME. 



Following the order of the synopsis a tew notes and references 

 will be in place. 



NOSTOC TENUISSIMUM, Kg. 



Scarcely separable from the next except that the thallus 

 is not so gelatinous a young condition of N. pruniforme. 



^OSTOC COMMINUTUM, Kg. 



Syn. N. piscinale. Kg. ; N. lacustre, Kg. ; N. agglutinum, Kg. 

 Floating in ditches and smaller ponds. 



NOSTOC SPONGI^FORME, Ag. 



NOSTOC ELLIPSOSPORUM (Desm. ) Eab. 

 Two forms not recognized. 



NOSTOC MUSCORUM, Ag. 



Usually found on wet calcareous rock, and on the mosses 

 which cover them. 



Plate CXCYII, fig. 35, a gelatinous thallus formed by the 

 confluence of a number of smaller subspherical thalli. 



NOSTOC HUMIFUSUM, Carm. 



A form described as globose, or subglobose, from the size 

 of a pepper corn ; often aggregated and confluent ; have not 

 identified it, but have suspected that the form alluded to 

 above as N. commune on Anastatia Island (p. 280) may be 

 this form. The same ground on which it occurs abounds 

 also in a small form of ftcytonema, near 8. chrysochlorum, Kg. 



