GrevillEj on New Diatoms. 
65 
lines are interrupted^ and here and there the puncta are 
either altogether wanting or look as if they had been 
shaken out of their places. At present it is impossible to 
say whether this centrical irregularity is accidental or other- 
wise. 
Porodiscus conicus, n. sp._, Grev. — Small; disc conical, 
unarmed, with an obtusely truncate apex; radiating lines 
of puncta extremely minute. Diameter '0014". (Fig. 3.) 
Hab. Barbadoes deposit from Cambridge estate ; C. John- 
son, Esq. ; rare. 
The smallest of the species hitherto discovered, and oc- 
curring not unfrequently in perfect frustules. The length of 
the connecting zone is considerable, and that of the entire 
frustule, when both valves are symmetrical, about -0040". 
The valve is decidedly conical, but obtusely truncate at the 
top when seen in profile. It hardly ever happens that the 
valves are equal in the same specimen ; indeed I do not think 
that I have seen a single example perfectly symmetrical, 
one valve being almost always considerably shorter than the 
other. The length of the connecting zone gives the frustule 
a cylindrical appearance. 
Porodiscus niiiduSj n. sp., Grev. — Disc convex, unarmed, 
the longest lines of puncta single (not in pairs), alternating 
with two or three series of shorter ones ; puncta distinct, all 
of them becoming much more minute towards the margin. 
Diameter '0026". (Fig. 4.) 
Hab. Barbadoes deposit, from Cambridge estate ; C. John- 
son, Esq. 
Disc much less crowded than in the three preceding species, 
and the puncta larger and more distinct. A certain number 
of the lines reach from the margin to the centre ; a second 
series very nearly so ; a third are considerably shorter, and 
the last extend but little beyond the margin. It is a scarce 
species. 
Porodiscus oblongus, n. sp., Grev. — Disc elliptical-ob- 
long; pseudo-opening large. Long diameter about -0028". 
(Pig. 5.) 
Hab. Barbadoes deposit. 
A species by no means rare in some specimens of the 
deposit which I investigated a few years ago, but it does not 
seem to occur in those which have recently been so carefully 
examined from Cambridge estate. The form alone is suffi- 
cient to identify it. The pseudo-opening is very large, the 
radiating lines of granules are less crowded, and the granules 
themselves larger than in any of the preceding species. 
