33 



Descriptions of New mid Rare Diatoms. Series I. 

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



(Read March 12th.) 



Stictodiscus_, n. gen., Gi'ev. 



Frustules simple, discoid, divided by radiating lines into 

 numerous plicate compartments. Lines not reaching the 

 centre. Compartments furnished with conspicuous transparent, 

 pore-like puncta. (In the four typical species, large scattered 

 puncta also occupy the blank central portion of the disc.) 



This genus is founded primarily upon Discoplea ? Rota 

 and D. ? Rotula of Ehrenberg, and two most beautiful dia- 

 toms which occur in a deposit found in the Island of 

 Trinidad. While enga2;ed in preparing a description of the 

 two latter, my friend, Mr. Ralfs, directed my attention to the 

 idea thrown out by Ehrenberg, that Actinoptychus dives, and 

 Cyclotella Rota, and C. Rotula might be generically associ- 

 ated ; and that they would come very conveniently into the 

 new genus I was proposing to establish. The words of 

 Ehrenberg are (under his definition o^ Discoplea? Rota) — • 

 " Proxime ad Actinojjtychicm divitem in Grsecia fossilem acce- 

 dens forma, et cum ea forsan, et cum sequente (Discoplea ? 

 Rotula) in peculiari genere reponenda.'^ ('Bericht. Berl. 

 Akad.,' 1844, p. 202). I entirely concur in this view. Four 

 of the species enumerated in this paper, namely, Siictodiscus 

 Buryanus, S. Johnsonianus, S. Rota, and S. Rotula, maj'^ 

 be considered typical, being distinguished not only by the 

 pore-like puncta or papillae, or whatever they may be called, 

 which occupy a definite (?) arrangement within the compart- 

 ments, but by large puncta remotely scattered over the con- 

 vex and otherwise blank centre of the disc. The remaining 

 species, which agree in general habit, and in the presence of 

 definitely arranged puncta or cellules within the compart- 

 ments, may be at least retained provisionally. 



For the discovery of the deposit in Trinidad, new, I believe, 

 to the microscopic world, we are indebted to Dr. John Davy, 

 well known for his researches in various departments of 

 natural history. He kindly informs me that, from his obser- 

 vations made in Trinidad, he is disposed to consider the 

 formation in which the deposit occurs as connected with the 

 New Red Sandstone ; adjoining to which is the sandstone, pro- 

 bablv of the same description, in which the Pitch Lake is 



