932 
THE VOYAGE OF H.M.S. CHALLENGER. 
similar to those which I have seen in preparations made at the moment of capture 
(see fig. 333). I have thus been able to determine four specific types of these gigantic 
Diatoms, which together with smaller forms, to which, however, they are not so closely 
related, constitute a new genus, which it will perhaps be necessary to divide into two. 
“ The discovery of forms so remarkable for their size and the tenuity of their walls 
may well lead us to wonder what new forms may be discovered in the order of the 
Diatomaceae, during the prosecution of researches into the unlimited extent of seas, 
in which such a noble example has been set by the British Government.” 
For the extremely large and delicately sculptured forms j ust alluded to, Count Castra- 
cane has proposed the generic name of Etmodiscus. In addition to the general features 
already quoted, this genus is characterised by a well-defined convexity of its valves, and, 
in some cases, by a great development of the connecting zone, which may proceed so far 
as to cause the axial line of the frustule to exceed the transverse diameter in length. 
Delicate radiating striations occur on the valves of several species, such as Etmodiscus 
radiatus, Etmodiscus coronatus, Etmodiscus humilis, &c., and a corona of great beauty 
and regularity is found in the form of distinct submarginal 
points in Etmodiscus punctiger, as radiating small lines in 
Etmodiscus japonicus, or as two alternate rows of granules in 
Etmodiscus coronatus. Sometimes, as in Etmodiscus punctiger, 
the entire surface of the valves present a finely punctated 
appearance, but in other cases granules of small size are to be 
met with only on subcentral and submarginal regions, the inter- 
mediate space being devoid of such markings as in Etmodiscus 
periachantinos, or on the submarginal area alone, as in Etmodiscus 
coronatus. In Etmodiscus convexus the large discoid al convex 
frustule exhibits an almost invisible striation “ seminated with 
very minute thorns,” and provided near the margin with a row 
of delicate “denticules” which differ from the corresponding 
submarginal circle of points found in Etmodiscus punctiger by being at once more 
numerous and more minute. 
The valves of Etmodiscus radiatus present a sculpturing of singular beauty, and one 
which recalls the appearance presented by the medullary rays of an ordinary dicotyledon- 
ous tree. Thus its submarginal region forms a belt of small irregularly disposed granules, 
from which several rows of different lengths proceed towards the centre. Of these rows 
none reach that point however, the shortest being about one-half, and the intermediate 
about three-fourths of the length of the longest series. The disposition of these rays on 
the surface of the valve is perfectly regular and symmetrical. From the central point 
very delicate striae proceed towards the periphery, but run across only about three- 
fourths of the diameter of the valve, the border of which is also striated. 
Fig. 332. — Frustule of Etmodiscus 
wyvilleanm, n. gen. et. sp. , 
