362 CONTRIBUTIONS PROM l ill NATIONAL HERBARIUM. 



GYROSIGMA H 



11 M • Bi \ le 1: 136. pi. 102./. a. 1845, Rabfa Sussw. Diat 

 emend. 8 ll 26 i 



tith, Ann t. Hist. I. pi. i . 1852. Bynop. Brit. Diat 



1: I 3 Rabh. II. Eur. Ug. 1: 230. 1864. Prifc 1. Hist farm ed. 



115. L861. Grun. Verh. Zool Wien 10: 555. I860. « leve A Grun. 



\k..d. Handl 17: 18 L880 Van Heur. Synop. 114. pi. 18-81. 1881; 

 Treat. Diat. 249. pi. 5-7. 28. 18 1 pi. / to. 



1891. ( astr Rep \ oy. Chall. I riff, a H< ed. 



1.006. pf. !'■■ L883. De Toni, Syll. Alg. 2: 231. L891. Cl< et \k..d. 



Bandl. 26- 112. L8 

 /' il ' ■< Vg. in part; Km/. Linnaea 8: 555 noe 19 50 . pi. / 



Bory, in pan: Ehrenb. [nfus. 180 181 n tS. f. to /.;. 



L838. Km/. Bacill. L02 no. 132 . pi. \.f.S2. L844. 

 midea Donk. Trans. Micr. Soc. Lond. n. b. 6: 19. pi. .. /'. ; /. L858. Clevi - 

 Ucad. Bandl. 26- 15. L894. Van Beur. Treat. Diat. 247. L896. Pritch. 

 Hist. [nfus. ed. !. 920. 1861. Perag. Le Diatomiste 1 : 27. pi. 9. 1891. 

 Donkinia RaJfa in Pritch. Hist Infus. ed. 1. 920. pi. S.f. />. L861. Van Heur. Treat 



Diat. 248. L896. Perag. Le Diatomiste V : 29. pi. 9. L891. 

 Rhoicosigma Grun. Hedwigia 6: L0. L867. Van Heur. Treat. Diat. 260. L890. 



I e Diatomiste V : 30. pi. 9 /". 

 This genus of diatoms is one of the best defined and mosl sbarply separated of the 

 large genera. Its valves are more or less sigmoid, both as to their outline and in resped 

 to the trend of the raphe. They are evenly marked with delicate Btriatl 

 beauty and often of extreme delicacy, either in two directions, transverse and longi- 

 tudinal, or in three directions, one transverse and two oblique, the three being at 

 angles approximately of 60 to each other. This Btriation covers the entire valve 

 with a gauzy network, except a very narrow longitudinal space t li r« uiltIi which runs 

 the raphe and a minute area around the central nodule. The valves are 

 rule convex, thin, made up of two or three layers, and therefore Btrongly prismatic. 

 The zonal view is in most cases extremely narrow compared to the > alval view. Their 

 beauty, wide distribution, and importance I the efficiency of microscope 



objectives have resulted in their being more generally known and carefully studied 

 than most other genera. 



William Smith's generic name, Pleurosigma, is very appropriate and has been uni- 

 versally accepted in preference to the earlier one of Hassall. But there can be no 

 question of the validity of Gyrosigma. We find it in his British Algae as a mono- 

 typic genus, based on G. hippocampa Ehrenb. Ha—., the description and illustration 

 of which are unmistakable. Hassall also quotes Ehrenberg's excellent reference to 

 this diatom, as Navicula hippocampus Ehrenb. The reasons advanced by Brebisson, 

 Ralfs, and others, for preferring Smith's name ; ire wholly inadequate; and although 

 we are indebted to Smith for a most clear and comprehensive analysis of this genua 

 ii is necessary to recognize the right of Hassall 's name to represent these diatoms. 

 Several attempts have been made to create new genera out of certain Bpe< 

 sigma having more or lese Btriking peculiarities. The firsl was the separation 

 of all Bpecies having an arcuate instead of sigmoid raphe, with valves extremely con- 

 vex "ii one aide and nearly straight on the other 1 i. ... strongly asymmetrical on their 

 longitudinal axis). These were placed in a genus, Toxonidia, bj Donkin.6 The 

 nexl separation was by Ralfs of valves with extremel) sigmoid raphe, the ridge of the 

 raphe being raised into a keel above the rest of the valve, except at the central nodule. 



Ehrenb. [nfus. 180. pi. IS.f. 11. l v 

 b Donk. Trans. Mici - I md.n. s. 6: 19. pi. S.f. 1. 1858. Moeb. Diat.-taf. pi. 18, 

 /". /. 1890. 



