930 



SYSTEMATIC HISTORY OF THE USTFUSOEIA. 



The fii'st contains species having series or files of naviculee surronnded by 

 a gelatinous covering, and by their union forming a compound, generally 

 stout frond, externally furnished with a common epidermis. Type, M. Smithii. 

 The species of this section are highly gelatinous, and consequently adhere 

 firmly to paper in drj^ing, are frequently of considerable thickness at the 

 base, and often have their extremities lobed, proliferous, or penicillate. The 

 margins, especially in old specimens, are generally more or less rough or 

 irregular. The frustules are released by the destruction of the gelatine, and 

 not by injmy to the extremities, as in the ease of tubular fronds. Each 

 series of frustules seems to have its own proper gelatinous covering, and the 

 junctions are marked by faint longitudinal lines; but these, which have 

 been supposed to indicate tubes, are often very indistinct ; hence arises much 

 of the difficulty in determining their proper genus. 



The second section contains species which have a strictly compound frond 

 of distinct tubes longitudinally connected, each tube similar to a frond of 

 Schizonema. The fronds are generally membranaceous, and adhere imper- 

 fectly to paper; the frustules, arranged more or less irregularly in their 

 tubes, are liable to escape from an opening at any part. Type, M. cornoides. 

 If these sections, as is probable, should hereafter rank as genera, the three 

 allied genera might be named and characterized as follows : — 



1. MoNis'EMA (Grev., Meneg.) (=Schizonema, Ag., Kutz.). — Frond tubular, 

 single-tubed. 



2. ScHizoKEMA (Ag., Grev., Meneg.) (=Micromega, Ag., Kiitz.). — Frond 

 gelatinous, not tubular. 



3. MicEOMEGA (Ag., Ehr.). — Frond tubular, two- or more tubed. The 

 branching in Micromega, especially in the species belonging to the first 

 section, results from the separation of the series of naviculae, and is not the 

 branching of a tube, as in Schizonema. 



* Frond gelatinous, containing longitu- 

 di?ial series of naviculcs. 



MiCEOMEGA Smithii (Ag.). — Frond 

 robust, setaceous, gelatinous, firm, simple 

 below, much branched above, frustules 

 in longitudinal series ; valves lanceolate, 

 acute; striae 40 in -001". = Schizmiema 

 SmWm, AD. p. 18; SD. ii. p. 76, pi. 57. 

 f. 362. Rocks and Algae. Common. 

 Fronds usually scattered, pale-yellowish 

 olive; naviculae disposed in subdistant 

 files within the colourless lelly of the 

 frond. 1-600". 



M. helminthosum (Chauv.). — Fronds 

 robust, setaceous, gelatinous, lubricous, 

 tufted, much branched, with acute apices; 

 naviculae in longitudinal series; valves 

 elliptic-oblong, with rounded ends ; stride 

 48 in -001". = Schizonema helminthosum, 

 AD. p. 20; KA. p. 103; SD. ii. p. 74, 

 pi. 56. f 355 ; S. Arhuscida, KB. pi. 27. 

 f. 1 ; Naunema Arhuscida, E Inf. ; N.fru- 

 ticidosum, KA. p. 104. Rocks. Colour 

 olive-brown, becoming greenish-grey and 

 without gloss in drying. 1-504". It 

 may be known froni M. Sjnithii by its 

 more tufted, often gregarious, lubricous 



and darker-colom'ed fronds, and its larger 

 and very obtuse valves. 



M. torquatum (Harv.). — Frond robust, 

 simple below, much divided above, ulti- 

 mate divisions much twisted; naviculae 

 in longitudinal files ; valves oblong-lan- 

 ceolate; striae 40 in 'OOV . = Schtzone7na 

 torquatum. Me. ; SD. ii. p. 76, pi. 57. 

 f. 361 ; aS'. Smithii, /3. torquatum, Harv. 

 Manual, p. 211; Micromega setaceum, y. 

 torquatum, KA. p. 107. Britain. In size 

 and colour it agrees with M. Smithii, but 

 is remarkable for having its branches 

 curled ; its na'viculae are more distinctly 

 in chains, shorter and broader in propor- 

 tion to their length than in that species. 

 1-720" to 1-690". 



M. Poly dados (K.). — Frond setaceous, 

 dichotomously branched ; branches elon- 

 gated, slender, rather rigid; naviculae 

 (membranaceous ?), flaccid, in distinct 

 tubes. KB. p. 118, pi. 28. f. 1. Europe. 

 Spermatia eUiptic. 1-1086" to 1-960". 

 Meneghini unites this form with M. tor- 

 quatum. 



M. nehidosiim (Me.). — Frond slightly 

 greenish, cloudy, subachromatic, forming 

 an intricate mucous stratum ; tubes gela- 

 tinous, achromatic, obsolete; naviculae 



