330 Proceedings of the Royal Irish Academy. 
Colletonema is in a twofold aspect uncertainly founded. On the one 
hand, it can scarcely be rightly separated from Schizonema, in which 
small forms occur in simple sheaths, and on the other hand its separa- 
tion from Navicula is very uncertain. It appears to me that many 
species of Navicule may, under certain conditions, occur, as well in 
gelatinous masses as inclosed in gelatinous tubes, 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 observations on 
Colletonema neglectum, he remarks: ‘‘ I once observed this species in 
an unused mill-stream in which Navicula gracilis occurred in uncommon 
abundance, and for the most part certainly in a free state; very fre- 
quently also were found gelatinous tubes filled with perfect frustules 
of Navicula gracilis, just as Smith has described it, and also very un- 
frequently bands consisting of double rows of the same Navicula 
without any sheaths; nor could I by the most careful examination dis- 
criminate between these forms and those of Navicula gracilis from 
other localities, where no gelatinous tubes were discovered.’’—Verhand. 
der K. K. Zool. Bot. Gesel., Band x., 1860, pp. 570, 571. These 
observations coincide with the supposition of Wm. Smith, that ‘‘ Pin- 
nularia radiosa may be merely a free state of Colletonema neglectum 
and Navicula crassinervia, the same condition of Colletonema vulgare.” 
B. D., Vol. u., p. 69. I take the opportunity of remarking that, in a 
gathering made by me from Lough Aron, on the summit of the Sheve- 
anieran mountain, Co. Antrim, in the summer of 1872, Navicula rhom- 
boides occurred in great abundance ; some of the forms were free and 
active, others were inclosed in gelatinous tubes, invariably arranged in 
single files, and by no means uncommonly the frustules 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 Navicula 
eracilis. Rabenhorst restores the species of this genus to Schizonema ; 
and Heiberg, rejecting the generic distinction founded on the gelati- 
nous tubes in which the frustules are invested, unites them with 
Navicula. 
Reproduction has been observed 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 frustules are frequently found in a filament intermixed 
with ordinary frustules, from which they differ only in size.’””—Ann. 
Nat. Hist., March, 1848. Pfitzer superadds, that ‘‘two cells produce 
two auxospores.”’—Untersuchungen, p. 73. 
Colletonema eximium, (Thwaites), Fresh water. 
Frond filiform, frustules arranged in one or more rows; valve 
sigmoid, strie fine, parallel. 
Rab. Siissw. Diat., p. 51. Wm. Sm., B.D., Vol. ii., p. 69, Pl. lvi., 
fig. 350. Ralfs, in Pritch., p. 926, Pl. vii., fig. 43. Grunow, Ver- 
hand. der K. K. Zool. Bot. Gesel., Band x., 1860, p. 573, who remarks, 
regarding this species: ‘‘it must either be transferred to Pleurosigma, 
