Greville^ on Asterolampra. 
105 
maudie/ 1857. The valve is " suborbicular/^ and the "point 
d^ou rayonnent les amhulacres .... est tonjours ex- 
centrique.^^ 
It will be perceived that if we take a frustule of Astero- 
lampra (which may be assumed as the typical form of the 
group), and merely approximate two of the umbilical lines, 
so as to become parallel, and make the ray between them 
narrower than the rest, we have at once an Aster omphalus ; 
and that between the latter genus and Spatangidium there is 
only (according to De Brebisson^s view) the eccentrical posi- 
tion of the hyaline area, with its lines and rays. Again, if 
we take the genus Asterolampra as our stand-point, and 
examine in what respects Ehrenberg has made Aster omphalus 
to differ from it, we are reduced to the conclusion that the 
former rests its claim to generic distinction, as compared 
with the latter, on the sole circumstance that the rays and 
umbilical lines are all equal. 
Let us now examine how far these genera have been 
affected by recent discoveries. In Asterolampra the species 
named Marylandica was the only one known to Ehrenberg ; 
and though various others have been subsequently described, 
they must all be referred to that species. In all of them the 
areolated segments are concave at the base, and the umbilical 
lines straight and simple. But other species, very different 
in habit, must now be introduced on the claim specified in 
the close of the last paragraph. One of these is Aster om- 
phalus Grevillii, of Wallich,^ from the Indian Ocean, found 
also by Mr. Dallas in the Rappahannock deposit. United 
States, and by Professor Walker- Arnott in Monterey stone ; 
in which the numerous segments are square at the base, and 
the umbilical lines are variously divided. In A. variabilis, 
another species found in the same material, the umbilical 
lines are most of them divided near to the central point, 
being either forked or arranged in triplets ; while the base of 
the segments is so sharply angular as to give a triangular 
aspect to the adjoining portion of the rays. 
In a third species from the same source {A. Brebissoniana, 
PI. Ill, fig. 9), the numerous segments are square at the base, 
and the umbilical lines exhibit the angular bend in the mid- 
dle, which is observable in certain species both of Asterom- 
phalus and Spatangidium. It is impossible not to perceive 
how strongly the characters and natural habit of the diatoms 
noticed above run into those of the last-named genera. 
Aster omphalus, as we have already seen, only differs (ac- 
* ' Traus. Mic. Soc.,' vol. viii, p. 47, PI. 2, fig. 16. 
