302 Proceedings of the Royal Irish Academy. 



Synedra hiceps, (Kiitz.) Fresli water. 



Prustules considerably larger than those of the last species ; on 

 front view quadrangular ; on side view arcuate ; extremities capitate, 

 (PL 28, fig. 16.) 



Kiitz. Bac, p. 66, T. xiv., figs. 18 and 21. Wm. Sm., B. D., VoL 

 i., p. 69, PI. xi., fig. 83. Rab. Slissw. Diat., p. 55, T. v., fig. 9. 

 Ralls, in Pritch., p. 786. Heiberg, De Danske Diat., p. 65. — 

 Synedra flexuosa ? Castracane, Catalogo di Diat. raccolte nell Val 

 Intrasca, p. 10. Rab. M. Eur., sect. 1, p. 129. — Synedra flexuosa, yar, 

 biceps, Grunow, Yerhand. dcr K. K. Zool. Bot., Gresel. Band xii.^ 

 1862, p. 390. 



Killikee, Co. Dublin. Carrickmacrilly, Co. Wicklow. Glencar^ 

 Co. Kerry. Conneniara, Co. Galway. Bantry, Co. Cork. 



(c.) Strice pervious, except in the middle, tchere there is a free space^ 

 'bounded ly a more or Jess perfectly developed riny. 



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 : Synedi^a 

 pulchella, Kiitz., freshwater; S. gracilis, Kiitz., brackish water ; S. 

 acicularis, "Wm. Sm., which he makes = S. leevis, Kiitz, brackish 

 water; and S. minutissima, Kutz., fresh water. The same author ex- 

 cludes from this sub-division S. 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- 

 gined 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, Kiitz., in Wm. Sm., B. D. Syn. 

 parvula, Kiitz., he regards as = Syn. fasciculata, Kutz., Wm. Sm., B.D. ; 

 and Syn. vaucherise, Kiitz., as = Syn. minutissima, Kiitz., in Wm. Sm., 

 B.D., as well as to Syn. vaucheriae, 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 



