WALKER-ARNOTT, ON RHABDONEMA. 91 
Annuli few, about 5 in ‘001, with about 50 striz in ‘001. 
Septa apparently opposite and rudimentary, or if present are 
not marked by coste or perforations. Valves very convex, 
arcuate, or somewhat linear and lunate, considerably broader 
than the annuli, the entire frustule presenting the appear- 
ance of a little bridge (Gephyria) with a low parapet on each 
side. Striz oblique, only between the costz, very much 
fainter than in Hu. pulchella, but not seemingly much more 
numerous (from 36 to 40 in :001), although requiring careful 
illumination and an object-glass of high power (3-inch) to 
exhibit them. 
In all these the median line or costa is not quite straight, but 
shghtly bent in a zigzag manner, the lateral ones being gene- 
rally alternate and proceeding from the angles of the median 
one. In the inferior valve the median line projects slightly 
beyond the lateral ones, and there usually forms a little 
knob. 
I have not attempted to measure the distances of the strize 
or annuli with accuracy; the numbers I have assigned are 
therefore more to be considered as relative, and probably are 
very different from what others may determine them to be. 
I shall now give the generic character of Rhabdonema 
slightly restricted from what is given by Smith, with abridged 
characters of all the known species. 
RuABpONEMA, Kutz. 
Filaments compressed, continuous, attached, or stipitate. 
Frustules annulate, indefinite ; annuli plane, cellulate on their 
circumference ; septa opposite or alternate. Valves similar, 
which is a genuine species of Amphiprora. This was first found copiously 
in the living state, and properly understood by Mr. Ralfs, of Penzance; and 
as the specific name given by Dr. Greville is quite inapplicable to an AmpAi- 
prora, \ have no hesitation in changing it to 4. Ralfsic. 
A. Ralfsii; F. V. narrow, elliptical, deeply constricted; V. twisted 
equally from end to end; striz about 53 in ‘001.—A. didyma, Sm. Brit. 
Diat., t. 15, f. 125? (excl. the name and char.)—Plewr. compactum, Grev., 
M. J. V., t. 3, f. 9 (mame and char. erroneous). 
Hab. Penzance; J. Ralfs. Cumbrae; R. Hennedy. Dredged also in 
various places in the Clyde by Mr. Hennedy and Rev. Mr. Miles. 
The figure given by Sinith is equally characteristic of this species, and 
of what Dr. Gregory calls 4. Lepidoptera; but if the number of strie, 72 
in 001, be not an error of the press for 52, it can be neither. Dr. Gre- 
ville describes the strize as “ obscure,” which they are under a bad object- 
glass; but they are as conspicuous and as few in ‘001 as in Pleur. istuarit, 
which usually in this country accompanies it, both in the normal and dis- 
torted shells; the latter of which has both ends bent in the same direction, 
a structure that occurs in some other species of that genius, 
