260 Proceedings of the Royal Irish Academy. | 
simply because in the latter the striation was, as he describes it, areo- 
late. But subsequent writers found this distinction untenable, inas- 
much as in Creswellia, connected with Melosira by the filamentous 
character of its frustules, the striation is distinctly areolate, while in 
some of the species which are properly included in the genus Coscino- 
discus the areolate character disappears. 
Heiberg is dissatisfied with the diagnoses which preceding authors 
have given, but in consequence of the limited amount of material for 
observation at his command, declines to attempt a more satisfactory 
definition. It appears to me that if Coscinodiscus excentricus, which is 
described as having a spinous or dentate margin, be excluded, we 
shall then have a tolerably well-marked group, as above defined. 
(a) Disk with a central rosette. 
Coscinodiscus oculus iridis, (Khr.) Marine. 
Central rosette, consisting of from six to nine large oblong cellules. 
Cellules large, hexagonal, radiate, distinctly smaller as they approach 
the margin. (Pl. 26, fig. 18.) - 
Ebr. Mic., T. xviii., fig. 49. Ralfs, in Pritch., p. 828. Raben. 
Fl. Eur., p. 34. Heiberg, De Danske Diat., p. 35. Cleve, Om Svenska 
och Norska Diat., p. 217. 
Tide pool, Monkstown; on sea-weeds, Ballybrack; tide pool, 
Dalkey ; Oyster-shells, Dublin Bay, all in the County Dublin. 
Coscinodiscus centralis, (EKhr.) Marine. 
Central rosette consisting of about eight large rounded cellules 
surrounding a single central one. Cellules distinctly hexagonal, 
radiate, nearly equal, and smaller than in the former species. (Pl. 26, 
fie. 19, 
i ae Mic., T. xviii., fig. 39. Greg. Diat. of Clyde. p. 28, Pl. xi., 
fig. 49. Ralfs, in Pritch., p. 828. If Gregory describes and figures 
with accuracy the form so named, and found by him in Glenshira 
Sand, as well as in the Clyde, it can scarcely be identical with the pre- 
sent species. The only difference, however, is in the character of the 
cellules forming the central rosette, which, in his form, consists of 
‘‘three large oblong cells meeting in a point, and between these, a 
little farther from the centre, three more cells, a little smaller.” 
Ralfs, however, as above cited, describes this portion of the valve as 
consisting of ‘(a few oblong cellules, round a circular one ;” which 
description accurately represents the appearance of the rosette in the 
present form, and therefore I adopt the specific name. 
On sea-weeds, Ballybrack, Dalkey, Co. Dublin. Stomachs of Asci- 
dians, Belfast Lough. 
