O'Meara — Rejjort on the Irish Diatomaccie. 251 



succeeding authors. Ralfs and Rabenliorst, however, abandoning the 

 generic distinction, have relegated the several species of Orthosira to 

 the genus Melosira. Heiberg, on the contrary, recognises the distinc- 

 tion of Thwaites, but includes the species of Cyclotella in the genus 

 Orthosira, and establishes a new genus, Paralia, to receive the single 

 species Orthosira marina (AYm. Sm.), on the ground that the frustule 

 possesses an elevated keel similar to that which characterises the 

 genus Lysigonium. It is a question, then, whether the distinction of 

 Thwaites should be recognised, as most authors since his time have 

 done, or discarded, as Rabenhorst has considered it ought to be ; and 

 the following observations of Pfitzer seem to supply a reasonable 

 solution. Having referred to the original distinction, he adds, "A 

 far more important distinction exists in the mode of developing Auxo- 

 spores. Although the Orthosirse in their mode of growth agree 

 thoroughly with Melosira, have the same structiire of the primordial 

 cell, and the same mode of cell-division, they differ in this respect, 

 that in the process of spore-fonnation from a single cell, the valves of 

 which are pushed away from one another, the contents enveloped in a 

 mucous investment come out free, and are then, without being in con- 

 tact with the mother-cell, developed^into a single Auxospore in which 

 the firstling cell is so situated that the plane of division crosses that of 

 the mother-cell, whereas in Melosira it is parallel to it. Thwaites 

 had observed this feature in the case of Orthosira aurichalcea, and 

 Smith refers to the peculiarity as an important generic distinction. 

 But only one species had been observed in this aspect, and so it was 

 questionable whether all the Orthosir^e obeyed the same law. Pr. 

 Schmitz has succeeded in proving this in respect to another species, 

 Orthosira roeseana (Rab.), = 0. spinosa (Grrev.) " TJeber Bau und 

 Entwicklung der Bac, p. 134. If then the mode of developing Aux- 

 ospores be regarded, as I consider it ought to be, of importance as a 

 generic distinction, the conclusion is inevitable that the genus Ortho- 

 sira should not be merged in Melosira, as Balfs and Eabenhorst have 

 treated it. And also, forasmuch as in those species of Cyclotella in 

 which the formation of Auxosporcs has been noticed, the daughter-cell 

 is parallel to the mother-cell, for this reason, as well as on the old 

 ground of distinction, the species of Cyclotella should not with Heiberg 

 be included in the genus Orthosira. 



Orthosira arenaria, (D. Moore.) Fresh water. 



Frustules very large ; cell-cavity sub-spherical. Spines on juncture 

 surfaces short, broad, and close. Striae, on side view punctate, 

 radiate, stronger at the margin, and loosing their radiate arrangement 

 as they approach the centre. Strite on front view punctate, trans- 

 verse. (PI. 26, fig. 6.) 



Wm. Sm., B. D., Vol. ii., p. 59, PI. Hi., fig. 334. Heiberg, De 

 Danske Diat., p. 31. Ealfs, in Ann. N. Hist., Yol. xii., p. 349, 

 PI. ix., fig. 4. — Melosira arenaria, Kiitz, Bac, p. 55, T. xxi., fig. 27. 

 Rab. Siissw. Diat., p. 14, T. ii., fig. 5. 



B. I. A. PROC, 8ER. II., VOL. II., SCIENCE. 2 L 



