TRANSACTIONS. 



Descriptions of New and Rare Diatoms, Series IV. 

 By R. K. Greville, LL.D., F.R.S.E., &c. 



(Read June 12th, 1861.) 



Stictodiscus. 



Stictodiscus Calif ornicus, n. sp., Grev. — Puncta equal, large, 

 in rows of a single series ; rays obscure, terminating in 

 conspicuous, linear-oblong bases within tlie broad margin; 

 central puncta somewhat remotely scattered. Diameter 

 •0038'. (PI. X, fig. 1.) 



Hab. Monterey stone. 



A genuine Stictodiscus, distinguished from S. Johnsonianus 

 (which it resembles in the puncta, being arranged in single 

 rows) by the obscure and much shorter rays, by the broad 

 margin, and linear-oblong bases of the rays. Although the 

 latter are decidedly obscure compared with the same parts in 

 the other species, a careful adjustment shows their presence, 

 as well as the anastomosing lines towards the centre, which 

 exist in S. Buryanus and S. Johnsonianus. When the sur- 

 face of the disc is exactly in focus, the puncta appear simple ; 

 but by slightly lowering the focus a pore becomes visible in 

 the middle of each punctum ; and on viewing the valve from 

 within, the pores are very conspicuous, and placed on the 

 summits of little circular convex cavities (plane on the outer 

 surface, convex on the inner surface, of the valve), strongly 

 resembling the discs in the woody fibre of the Coniferse, 

 which are themselves little, plano-convex boxes, with an 

 orifice. The border of the disc is bounded by a row of 

 minute puncta. The number of rays is upwards of forty. 



Stictodiscus Kittonianus, n. sp., Grev. — Disc umbonate, 

 with a central nucleus ; rays numerous ; puncta minute, 

 equal, forming a double series in each compartment, and 

 closely covering the central space. Diameter about '0020". 

 (Figs: 2, 3.) 



VOL. IX. g 



