39 
Descriptions of New and Rare Diatoms. Series I. 
By R. K. Greville, LL.D., F.R.S.E., &c. 
(Eead March 12th.) 
Stictodiscus^ n. gen._, Grev. 
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 engao'ed in preparing a description of the 
two lattsr, my friend, Mr. Ralfs, directed my attention to the 
idea thrown out by Ehrenberg, that Actinoptychus diveSj and 
Cyclotella Rota, and C. Rotula might be generically associ- 
ated j and that they would come very conveniently into the 
new genus I was proposing to establish. The words of 
Ehrenberg are (under his definition oi Discoplea? Rota) — 
^' Proxime ad Actinoptychum 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 
Bury anus, S. Johnsonianus, S. Rota, and S. Rotula, may 
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- 
bably of the same description, in which the Pitch Lake is 
