Mr. J. Ralfs on the Nostochinere. 337 



a definite gelatinous sheath; ordinary cells subspherical, com- 

 pressed, about as long as wide ; vesicular cells subspherical, some- 

 what barrel-shaped, half as wide again as the ordinary cells, puncta 

 very distinct ; sporangia cylindrical, very unequal in length, and 

 with the ends rounded and somewhat truncated. 



Occurring amongst other algae from a freshwater boggy pool at 

 Wareham, Dorsetshire, Rev. TV. Smith. 



D. Smithii is immediately distinguishable from its congeners 

 on account of its possessing a definite gelatinous sheath to 

 each of its filaments, which are of smaller diameter than those 

 of any other species of Dolichospermum we are now describing. 

 The ordinary cells are subspherical, somewhat compressed, and 

 of less diameter than the vesicular cells, which are barrel-shaped 

 and with very distinct puncta. The numerous sporangia, which 

 are of about twice the diameter of the ordinary cells, are elon- 

 gated and cylindrical, very variable in length and in the number 

 which occur together, and their ends are slightly truncate. 



Plate IX. fig. 4. 



5. 1). Thwaitesii ( ). Filaments straight or nearly so ; ordinary 



cells quadrate ; vesicular cells oblong, subquadrate, puncta very di- 

 stinct ; sporangia numerous, cylindrical, with truncated ends, very 

 variable in length. Sphcerozyga Ral/sii, Thwaites in lit. (1849). 



In a freshwater pool, Dendham Down near Bristol ; also in a brack- 

 ish ditch near Shirehampton, G. H. K. Thwaites. 



D. Thwaitesii is nearly allied to the foregoing species, but its 

 filaments are not included in a definite gelatinous sheath. Its 

 filaments are also stouter than those of D. Smithii, and there is 

 a difference in the form of its ordinary as well as of its vesicular 

 cells. The vesicular cells of D. Thwaitesii are quadrangular, and 

 hardly exceed in diameter the ordinary cells. The cylindrical 

 truncated sporangia are numerous, occurring many in a chain, 

 and very variable in their length ; they are of about twice the 

 diameter of the ordinary cells. 



Plate IX. fig. 5. 



Cylindrospermum, Kutzing. 



Filaments simple, jointed, nidulating in a gelatinous stratum ; 

 vesicular cells terminal ; sporangia oblong or elliptic, inter- 

 posed between the vesicular and ordinary cells. 

 (Anabaina, Bory and others.) 



In Cylindrospermum the stratum is similar to that described 

 under the preceding genera ; but as the filaments radiate less 

 than is usually the case in Sphcerozyga, I was, in one instance of 

 admixture, enabled to separate the Spharozyga from the Cylin- 



Ann. &$ Mag. N. Hist. Ser.2. Vol. v. 22 



