DoNKiN^ on Marine Diatomacece. 
31 
in this group, is apparently developed_, only, in tlie hoop of the 
dorsal surface of the frustule, which seems to be constructed, 
like the bottom of a flattish boat, of several narrow longitu- 
dinal segments, which, like deals, are placed; edgewise with 
their extremities convergent. This structure is well seen in 
fig. 16 b, PI. Ill, and still better in A. Grevilliana, Greg., and 
A. spectabilis, Greg. The complex structure then is only 
observed as Professor Gregory has already pointed out " when 
the frustule is in a particular focus.^^ 
Hab. Chibburn Mouth, Druridge Bay ; abundant. 
16. A. arenai^ia, n. sp. — Frustule hyaline, colourless. 
F. V. rectangular; extremities slightly rounded ; sides some- 
what uneven, slightly bulged out in the middle and at the 
extremities. Length from 0*004" to 0-006"; breadth about 
0*0016 '. S. V. convex, linear, dorsal margin rounded, near 
the extremities, towards the apices situated on the ventral 
margin. Central nodule some distance from the inner mar- 
gin j longitudinal line much curved, first towards the dorsal, 
then near the extremities, to the ventral margin, where it 
joins the terminal nodules. 
The F. V. presents a space of the shape of a sand-glass 
between the two gracefully curved longitudinal lines. This 
space, when the dorsal surface is in focus, is faintly marked 
with from six to eight longitudinal lines; the outer con- 
verging at their extremities. These indicate a complex 
structure of the frustule. 
This large and interesting form is exceedingly hyaline and 
transparent when mounted in balsam. Mr. Shadbolt^ who 
carefully examined specimens with a very high power, informs 
me that "it (the S. V.) is ornamented with markings of dots 
at right angles to the axis as well as parallel thereto ; but 
these are of the most delicate nature, and discoverable with 
difficulty under the most careful manipulation with a ^ ob- 
jective of 165° aperture, and with peculiar illumination by 
the achromatic condenser.^^ 
The living frustule is remarkably beautiful ; the whole 
being filled with endochrome, having a greenish apppearance, 
and collected in difterent places into large, bright-yellow 
globules. In the water the ventral surface is always upper- 
most. 
Hab. Common along the southern portion of the Northum- 
brian shore. At Cresswell remarkably abundant, forming 
at least seventy parts in one hundred of the whole chestnut- 
coloured, diatomaceous mass with which the sinuosities in 
the sand, at low water, are covered. 
