160 GREVILLE, ON DIATOMACKEA. 
In accordance with the views which I have expressed, I 
now propose the following modification of the generic charae- 
ter of Asteromphalus. 
AstrrompPHaLus, Ehr.—Frustules simple, two-valved, disci- 
form, finely granulated; each valve marked with acuminated © 
rays, proceeding from a centrical or excentrical hyaline area, 
the median ray very narrow. 
I would here place the species described by Ehrenberg and 
Bailey, and also Spatangidum flabellatum and S. peliatum 
of De Brébisson. ‘There is, in fact, no character what- 
ever to separate the last two from this genus, except the 
excentrical hyaline area, which, as I have already stated, is 
sometimes an extremely ambiguous distinction. The Aste- 
romphali are very hyaline and delicate, the rays acuminate, 
and the structure minutely granulate. In natural hahit 
they all agree. I have detected in Californian guano cer- 
tainly two species; one new, the other possibly a variety of 
A. flabellatus; but I speak on this point with considerable 
hesitation. Indeed, I find it difficult to draw a satisfactory 
line between A. flabellatus and A. peltatus. The former, 
according to De Brébisson, has some of the rays subarcuate. 
The specimen (fig. 4) appears to be identical, only the 
rays are all straight. I have also represented, at fig. 5, 
another more perplexing form. It will be perceived that it 
is nearly quite orbicular, and that the hyaline area is very 
nearly centrical, and, as in the last illustration, the rays are 
all straight. I cannot, however, see how this can be sepa- 
rated from A. flabellatus, because the previous variety seems 
to form the connecting link. I may here mention, that I 
have a beautiful frustule, origm unknown, ovate in outline, 
with a very excentrical hyaline area, and eight rays, one of 
them short, and opposite to the median ray, as in A. peltatus ; 
the rest decidedly arcuate. If the number of rays be imma- 
terial, this will also have to be referred to A. flabeliatus, as 
well as another now before me with nine rays. In two spe- 
cimens of a fine species obtained from South African guano, 
the frustules of which are very nearly orbicular, and the 
hyaline area not far from being centrical, there are ten rays, all 
of which are straight; but the two lower lateral lines of the 
hyaline area on each side are arched downwards (the median 
line being directed towards the spectator). The granulation 
of the disc is also larger. This is probably a distinct species. 
I would not venture, im the absence of authentic examples, 
to say anything further regarding A. peltatus. ‘The short 
ray being in the same direction with the median ray and 
