68 Greville, on New Diaiome. 



This remarkable genus appears to belong to the group re- 

 presented by Asterolampra, but differs essentially in the ab- 

 sence of umbilical lines and of the hyaline area, so conspicuous 

 in all the discs referred to that genus. In the only known 

 species of the genus now proposed, the valve is cellulose, very 

 much in the manner of Coscinodiscus radiaius ; and the rays, 

 two in number, have their dilated bases separated by a con- 

 siderable interval, and have no connection whatever with each 

 other. This singular diatom is worthily dedicated to my friend 

 Thomas George Rylands, Esq., of Heath House, Warrington, 

 a very acute observer, who communicated it to me soon after 

 its discovery by Mr. lialfs. 



Rylandsia biradiata, n. sp., Grev. (PI. VIII, fig. 1). 



Hab. Barbadoes deposit, very rare ; John Ralfs, Esq., T. 

 G. Rylands, Esq., Dr. Greville. 



A considerable number of specimens of this curious disc 

 have now been seen, and it is satisfactory to know that it is 

 quite constant to its characters. The cellules in the centre of 

 the valve between the bases of the rays are large ; they then 

 suddenly become smaller, and decrease gradually in size as 

 they radiate to the circumference. The rays are broadly 

 cuneate at the base, and linear as they reach the margin ; 

 they appear to be tubular, as in Asterolampra, and a faint 

 shadow indicates the continuance of this structure through 

 the middle of the dilated bases. In a single instance the two 

 valves occurred in situ, the rays of the lower crossing those of 

 the upper valve. 



COSCINODTSCUS. 



Coscinodicus symmetricus, n. sp.,Grev. — Granules radiating, 

 distinct, all equal and equidistant ; seven of the radiating 

 lines extending from the centrical granule to the circum- 

 ference; margin striated. Granules 10 in '100'' ; marginal 

 striffi 15 in -001". Diameter -0031". (PL YIII, fig. 2.) 



Hab. Barbadoes deposit; excessively rare. 



One of the most beautiful of the granuliferous group of 

 Coscinodisci, and well characterised by the equal distribution 

 of the granules. It is also distinguished by the manner in 

 which the radiating lines are arranged. From the central 

 granule proceed seven long lines, and within the compart- 

 ments so formed the next two longest are disposed, one on each 

 side, so as to form two equal sides of the triangle, and so on 

 until the wh9le space is filled up. 



Cresswellia. 

 Creswellia superba, n. sp., Grev. — Valves hemispherical. 



