ARCHER, ON A NEW SPECIES OF COSMARIUM. 249 



and acute, as in this species. From S. dejectum, Breb., it 

 may be known by its lanceolate, not elliptic, segments, and 

 its merely apiculate, not spinous, extremities. Its lanceolate, 

 pointed, not elliptic or reniform, segments separate this 

 species from S. brevispina, Breb., while the mucrones of the 

 latter are rather larger, though not more conspicuous. 



Genus — Closterium, Nitzsch. 

 Closterium directum, sp. nov. 



Specific characters. — Frond rather slender, scarcely curved, 

 nearly straight, linear, ends truncate ; fillets indistinct ; empty 

 frond, very finely and closely striate. 



Locality. — Several pools, Dublin mountains. 



General descripition. — Frond rather slender, about fifteen 

 to twenty times longer than broad, scarcely tapering, lower 

 margin very nearly straight, upper also very nearly straight, 

 but slightly depressed towards the truncate ends, giving the 

 frond a nearly straight outline. Endochrome with indistinct 

 fillets and a conspicuous series of large granules. Empty 

 frond generally colourless, sometimes faintly reddish near 

 the ends, very finely striate, the latter character sometimes 

 difiicult of detection in mounted, but quite evident in fresh, 

 specimens. Sporangium unknown. 



Occasionally a distorted specimen is met with, slightly geni- 

 culately bent, or a segment sometimes presents a somewhat 

 irregularly curved form ; but a similar circumstance is not 

 unfrequently to be noticed in Docidium Ehrenberghii, which 

 is a straight form. 



Measurements. — Length of frond, gV to 3^- ; breadth, ttVo 

 of an inch. 



Plate XII, fig. 23, frond with endochrome ; fig. 24, empty 

 frond. Magnified 200 diameters. 



Affinities and differences. — This species is decidedly the 

 straightest and most linear in form of all the genus Clo- 

 sterium, and it will therefore be necessary to contrast it with 

 those only whose curvature and tapering are the most slight. 



It may be advisable to contrast this species with C. didy- 

 motocmn, Corda, C. obtusum, Breb., C. amblyonema, Ehr., 

 C intermedium, Balfs, C. angustatum, Kiitz., and perhaps 

 C. juncidum, Ralfs, and C. gracile, Breb. This species 

 agi'ces with C. didymotocum iii its nearly straight frond and 

 truncate ends, but they diff'er in the following particulars : — • 

 In the former the frond is far more slender than in the latter, 

 which is stout, and in the former the upper and lower margins 



VOL. II. NEW SER. S 



