92 WALKER-ARNOTT, ON RIABDONEMA. 
elliptical, ecostate, with a median line, striate. Stric trans- 
verse, moniliform. 
Sect. I. Septa entire. 
1. Rh. arcuatum, Kutz. ; septa opposite; striz not reaching 
to the end of the valves. 
Hab. Shores of Europe, Africa, and North America. 
To this belongs Tessel/a catena of Ehrenberg. 
2. Rh. minutum, Kutz.; septa alternate; strize reaching to 
the extremities of the valves. 
Hab. Shores of Europe and North America. 
To this belongs Tessella catena of Ralfs. 
Secr. II. Septa alternate, with perforations along the middle 
between the margin and central aperture. 
3. Rh. Adriaticum, Kutz.; septa with one oval perforation. 
Hab. Shores of Europe, North America, Asia Minor, 
Mauritius, and Ceylon. 
Probably more diffused than any other species. To it 
belongs Tessella catena of Bailey. 
4. Rh. mirificum, W. Sm.; septa with several (3 to 12) 
irregular perforations. 
Hab. Shores of Ceylon and Mauritius. 
In this species the openings in the septa are irregular in 
form, and reach from the upper to the lower margin of the 
annulus ; while the portions of the septa that separate these 
are narrow, and resemble bars which are either straight 
across or slightly oblique. When two such septa cohere, the 
perforations seem to extend from the one end to the other, 
and are then in no instance, as far as I have seen, fewer than 
seven, including the central opening, but usually are much 
more numerous (20 to 25). 
In my preliminary remarks I have alluded to Achnanthes. 
I may here take this opportunity of stating that A. brevipes 
of Kutzing, which has a rather short stipes and valves with 
acute extremities, has been found lately (November, 1857), 
by Mr. Okeden, at Neyland, in South Wales. This is re- 
ferred by Smith to A. longipes, on account of the presence of 
costz on the valves, along with moniliform striz: to this 
there is no valid objection; but, in that case, it ought to be 
noticed as a well-marked variety, and the word “ obtuse,” in 
reference to the valves, deleted from the specific character, 
as by no means applicable to this form of the species. Mr. 
Okeden has also found (March, 1857) at Neyland the A. 
parvula of Kutzing! a species which must be removed from 
