302 Proceedings of the Royal Irish Academy. 
Synedra biceps, (Kiutz.) Fresh water. 
Frustules considerably larger than those of the last species; on 
front view quadrangular; on side view arcuate; extremities capitate. 
(PI, 28, te." 16.) 
Kutz. Bac., ~.-66, T. xiv., figs. 18-and 21: Wimiem.. B,D. vol, 
i., p-69, Pl. x., fig. 85. \ Rab. Sussw. Diat pool ve, mene: 
Ralts, in Pritch., p. 786. Heiberg, De; Danske Diat.).p. 65,— 
Synedra flexuosa ? Castracane, Catalogo di Diat. raccolte nell Val 
Intrasca, p.10. Rab. Fl. Eur., sect. 1, p. 129.—Synedra flexuosa, var. 
biceps, Grunow, Verhand. der K. K. Zool. Bot., Gesel. Band xii., 
1862, p. 390. 
Killikee, Co. Dublin. Carrickmacrilly, Co. Wicklow. Glencar, 
Co. Kerry. Connemara, Co. Galway. Bantry, Co. Cork. 
(c.) Strie pervious, except in the middle, where there is a free space, 
bounded by a more or less perfectly developed ring. 
The number of forms legitimately included in this sub-division is 
very limited, and still there is none, perhaps, in which greater con- 
fusion reigns. Smith includes the four following species: Synedra 
pulchella, Kiitz., fresh water; S. gracilis, Kiitz., brackish water; S. 
acicularis, Wm. Sm., which he makes =8. levis, Kitz, brackish 
water; and §. minutissima, Kutz., fresh water. The same author ex- 
cludes from this sub-division 8. fasciculata, which seems really to 
belong to it, judging from the description given, as well as from the 
figure. 
To look at the figures of these several species, it might be ima- 
eined there would no difficulty in distinguishing the one from the 
other; but, practically, the difficulty of determining is found to be 
considerable. Kiitzing’s figures of them are too vague, and his de- 
scriptions too indefinite, to help the student out of the difficulty. 
Grunow regards Syn. fasciculata, Kiitz., as= Syn. Saxonica of the 
same author and Syn. gracilis, Kutz., in Wm. Sm., B. D. Syn. 
parvula, Kiitz., he regards as = Syn. fasciculata, Kiitz., Wm. Sm., B.D.; 
and Syn. vaucherie, Kiitz., as = Syn. minutissima, Kiitz., in Wm. Sm., 
B. D., as well as to Syn. vaucherie, Kiitz.,in Wm. Sm., B.D. The 
habitat to which these forms have been respectively assigned will fur- 
nish no satisfactory distinction. Some are attributed to fresh water, 
some to brackish; but when forms are ascribed to the latter, it is diffi- 
cult to ascertain whether they are fresh water forms which have been 
carried down, or marine forms which have been carried up, or forms 
incidental to brackish water. Grunow has found Syn. fasciculata in 
salt water as well as in brackish, and likewise in the Franzensbad 
deposit, which is a fresh water deposit, and in which I have also 
found the form so named by that author; from this last fact, I con- 
clude that the form is essentially a fresh water one, and not therefore 
