182 Transactions of the Royal Microscopical Society. 
form in the valves being more or less constricted in the centre, and 
the median line is more accurate. Honduras, Quamero. 
R. compaction ( = Fleur osiyma compaction Greville), very 
variable in outline, differs from R. Reichardtii by the very curved 
median line. Not rare. Honduras, Campeachy Bay, Corsica, Quar- 
nero, Honolulu, Tahiti, Gallopagos, &c. 
The other species of Rhoicosigma are — 
R. ardicum Cleve, * Bihang. till. Vetensk. Akad. Handl.,’ 
Bd. I., No. 13, tab. iii., fig. 16. Arctic Sea, Finmark. 
R. mediterraneum Cleve, MSS. Yalve narrow, lanceolate ; 
median line very much curved; strife transverse; 17 in - 01 mm. 
Balearic Isles. 
It. falcatum (Donkin) = Pleurosigma falcatum Donkin. 
‘Q. Mic. Jour.,’ vol. i., n. s., pi. i., fig. 1. Cresswell and Boulmar 
Bay. 
Two or three other species have been imperfectly observed. 
Ceratoneis spiralis Kg., * Bacill.,’ II. 31, may possibly be a species of 
Rhoicosigma, but I have never seen anything like Kiitzing’s figure. 
Isthmia. 
I. Lindigiana, Grunow et Eulenstein. I. gracilis ecostata, 
valvis inequadibus, inferiore longiore, oblique conica obtusa, supe- 
riore breviore, oblique subtriangulari, margine superiore convexo, 
plus minus distincte bigibbo, valvarum margine inferiore serie 
annulare corpusculorum clavaeformium (interdum etiam irregulariter 
in cetera valvse parte interna distributorum) ornato, areolis in 
utriusque valvae dorso sitis ceteris majoribus, irregularibus, reliquis 
membraneum connectivam versus in lineas longitudinales curvatas 
ordinatis, apicem versus minoribus hexagonis et irregulariter dis- 
positis, membrana connectiva . lata, areolis in lineas longitudinales 
ordinatis ornata, in media parte minoribus, valvas versus elongatis. 
Longit. frustul. • 023- • 039 mm., latit. * 5- • 10 mm. Tab. CXCVI., 
Fig. I, a, h, c, d. 
Common in the Honduras gathering. 
This species is well characterized by the small club-shaped 
corpuscles in the interior of the valves. I have seen nothing 
analogous to these in any other diatom. The areolae are covered 
with minute dots when seen under a higher power. 
I. Lindigiana is very nearly related to an Isthmia inhabiting 
the southern seas, and seems there to represent the I. enervis of 
the northern seas. I name it I. ( enervis var. ?) capensis. It is 
not rare at the Cape of Good Hope, and it also occurs on the coast 
of Australia and the Polynesian Islands. It is distinguished from 
I. Lindigiana by the absence of the peculiar club-shaped cor- 
puscles, and from I. enervis by its narrower and longer valves. 
