330 Proceedings of the Royal Irish Academy. 



Colletonema is in a twofold aspect ■ancertainly founded. On tlie one 

 hand, it can scarcely he rightly separated from Schizonema, in which 

 small forms occur in simple sheaths, and on the other hand its separa- 

 tion from IS'aTicula is very uncertain. It appears to me that many 

 species of Kayiculas may, under certain conditions, occui', as well in 

 gelatinous masses as inclosed in gelatinous tuhes, and two of the- 

 forms which I have with some hesitation placed in this genus appear 

 to me to confirm this impression." Farther on, in his obserTations on 

 Colletonema neglectum, he remarks : "I once ohserved this species in 

 an unused mill-stream in which IS^avicula gracilis occuiTed in uncommon 

 abundance, and for the most part certainly in a fi'ee state ; veiy fre- 

 qacntly also were found gelatinous tubes filled with perfect frustrd.es 

 of Xavicula gracilis, just as Smith has described it, and also very un- 

 iTequently bands consisting of double rows of the same I^avicula 

 without any sheaths ; nor could I by the most careful examination dis- 

 criminate between these foi-ms and those of Kavicula gracilis from 

 other localities, where no gelatinous tubes were discovered." — Yerhand. 

 der K. K. Zool. Bot. Gesel., Band x., 1860, pp. 570, 571. These 

 observations coincide with the supposition of "Wm. Smith, that '' Pin- 

 malaria radiosa may be merely a fi'ee state of Colletonema neglectum 

 and Kavicula crassinervia, the same condition of Colletonema vulgare." 

 B. D., Yol. ii., p. 69. I take the opportunity of remarking that, in a 

 gatheiing made by me fi'om Lough Aron, on the summit of the Slieve- 

 anieran moimtain, Co. Antrim, in the summer of 1872, IS'avicula rhom- 

 boides occuiTed in great abundance ; some of the forms were fi'ee and 

 active, others were inclosed in gelatinous tubes, invariably arranged in 

 single files, and by no means uncommonly the fiiistules were seen in 

 long files, attached apparently one to another by the ends, without the 

 slightest appearance of tubes, just as in Grunow's case of !N^avicida. 

 gi'acilis. ilabenhorst restores the species of this genus to Schizonema ; 

 and Heiberg, rejecting the generic distinction founded on the gelati- 

 nous tubes in which the fnistules are invested, unites them with 

 Is^avicula. 



Beproduction has been obseiwed by Thwaites in the case of Colle- 

 tonema subcoherens ; he says: "The Sporangia of this species are 

 produced by the conjugation of a pair of frustules outside the filaments ; 

 but sporangial fiTistules are frequently found in a filament intermixed 

 with ordinaiy frustules, fi'om which they differ only in size." — Ann. 

 IS'at. Hist., March, 1848. Pfitzer superadds, that "two cells produce 

 two ausospores." — Tntcrsuchungen, p. 73. 



Colletonema eximiuni, (Thwaites), Fresh water. 

 Frond filiform, fi-ustules arranged in one or more rows ; valve 

 sigmoid, striae fine, parallel. 



" Eab. Siissw. Diat., p. 51. Y'm. Sm., B. D., Yol. ii., p. 69, PI. Ivi., 

 fig. 350. Ealfs, in Pritch., p. 926, PL viii., fig. 43. Grunow, Yer- 

 hand. der K. K. Zool. Bot. Gesel., Band x., 1860, p. 573, who remarks^ 

 regarding this species : "it must either be transferred to Pleurosigma;^ 



