TRANSACTIONS. 
Descriptions of New and Rare Diatoms. Series XIII. 
By R. K. Greville, LL.D., F.R.S.E., &c. 
(Commuiiicaf cd by F. C. S, Roper, Esq., F.L.S.) 
(Read May 11th, 1864.) 
(Plates XII & XIIT.) 
AULACODISCUS. 
Aulacodiscus extans, n. sp., Grev. — Large; rays 4, forming 
elevated ridges terminating in broadly rounded marginal 
inflations ; processes oblong ; granules small ; minute raised 
points remotely scattered over the surface of the disc. 
Diameter -0090". (PI. XII, fig. 1.) 
Hab, Barbadoes deposit, Cambridge estate; C. Johnson, 
Esq. ; R. K. G. ; extremely rare. 
A noble species, which for a long time I only knew by 
small fragments. It is conspicuous at once for the large 
cruciform ridges, which are, in fact, inflations of the disc 
extending from the centre to the circumference, and pre- 
serving a nearly horizontal position, while the intermediate 
spaces following the usual convexity of the disc pass out of 
focus. In the centre is a small circular blank space from 
whence the close lines of minute granules radiate, having a 
great similarity to those of A. decorous. 
The processes are so marginal that when the disc is viewed 
vertically, they reach or pass slightly beyond the outer line. 
Over the whole surface of the disc minute apiculi are remotely 
scattered, but are most evident on the large terminal in- 
flations. This fine diatom is allied to A. mammosuSj in which 
the inflations are also dotted Avith apiculi, which were over- 
looked in the representation. That species, however, is 
much smaller, and the inflations are suddenly elevated like a 
VOL. XII. h 
