88 
Edwards^ on Diatoynacece. 
the common A. undulatus of the British and American coasts. 
The other is found in Peruvian guano, and appears on 
S.V. very much like the former; but the segments either 
are not at all, or only slightly undulate. Each valve of 
this species is composed of two plates ; an interior, having 
coarse markings, and an exterior with fine markings. This 
exterior plate has also projections similar to those in true 
Eupodisci, and situated near the margin of the valve, and at 
a point midway between the divisions. The species seems 
to have been taken for A. undulatus ^ and may be a variety, 
though it seems to be distinct. Ehrenberg^s Heliopelta 
probably belongs to this genus, but as it has not been seen 
in the living state, it is unsafe to place it therein. All the 
other so-called species of Actinocyclus are totally distinct, 
and have their rays arranged as Shadbolt has described his 
genus Actinophcenia ; they in fact belong to that genus, and, 
in my opinion, to one species, the number of rays not being 
sufficient to warrant their division into species. Mr. Roper's 
Actinocyclus triradiatus I consider to belong to Coscinodiscus. 
The genus Actinoptychus of Ehrenberg has no undulations in 
the valve similar to Actinocyclus, but is either plain, with 
a convex edge, or convex at the edge and having a raised ring 
within it at variable distances from the centre. Whether 
this be a fact or not, I am uncertain, as the appearance that 
seems to indicate the existence of a ring, may result from the 
thickening of the valve at that point, as seems to be the case 
in Actinocyclus triradiatus. The markings in Actinoptychus 
appear in some species to be circular, but no doubt, under a 
high power, would be found to be hexagonal. They are 
arranged as follows : a certain number of lines of equal 
hexagons run, at equal distances apart, from the centre 
(which is generally plain) to the circumference — these we will 
call primary rays.'' Between each two primary rays is a 
" secondary ray,'^ not quite reaching to the centre, but 
reaching the circumference. Parallel to the secondary, and 
therefore not 'parallel to the primary rays, are lines of hexa- 
gonal markings similar to the rays, which vary in number, 
but are generally about four or five in number on each side 
of each secondary ray. These lines reach to the circum- 
ference, but are not as long as the secondary rays, on account 
of the interposition of the primary rays, and diminish in 
length as they recede from the secondary rays, so that the 
last is often formed of but two or three hexagons. Now, 
Smithes Eupodiscus fulvus and crassus differ from this only 
in having a pseudo-nodule just within the margin. This 
cannot, however, entitle them to rank with Eupodiscus argus 
