260 Proceedings of the Royal Irish Academy. 



simply because in the latter the striation Tvas, as he describes it, areo- 

 late. But subsequent TTriters found this distinction untenable, inas- 

 much as in Creswellia, connected with Melosii-a 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) Dish with a central rosette. 



Coscinodiscus ocidus iridis, (Ehr.) Marine. 



Central rosette, consisting of fi'om six to nine large oblong cellules. 

 Cellules large, hexagonal, radiate, distinctly smaller as they approach 

 the margin. (PI. 26, fig. 18.) 



Ehr. Mic, T. xviii., fig. 49. Ealfs, in Pritch., p. 828. Eaben. 

 Fl. Eur., p. 34. Heiberg, De Danske Diat., p. 35. Cleve, Om Svenska 

 och jS^orska Diat., p. 217. 



Tide pool, Monkstown ; on sea-weeds, Ballybrack ; tide pool, 

 Dalkey ; Oyster-shells, Dublin Bay, all in the County Dublin. 



Coscifiodiscus centralis, (Ehr.) Maiine. 



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. (PI. 26, 

 fig. 19.) 



Ehr. Mic, T. xviii., fig. 39. Greg. Diat. of Clyde, p. 28, PI. xi., 

 fig. 49. Ealfs, in Pritch., p. 828. If Gregory describes and figui-es 

 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 fi'om the centre, three more cells, a little smaller." 

 Ealfs, however, as above cited, describes this portion of the valve as 

 consisting of "a few oblong cellules, round a circular one;" which 

 description accui-ately represents the appearance of the rosette in the 

 present form, and therefore I adopt the specific name. 



On sea-weeds, Ballybrack, Dalkey, Co. Diiblin. Stomachs of Asci- 

 dians, Belfast Lough. 



