158 GREVILLE, ON DIATOMACE. ° 
species, all collected in the Antarctic regions during the 
voyage of H.M. Discovery ships Erebus and Terror, by Dr. 
Joseph Hooker, the talented and imdefatigable naturalist of 
the expedition. The specific differences are founded by 
Ehrenberg, and subsequently by Kutzing (‘Sp. Alg., 1849), 
on the number of “marginal rays” and on the straight or 
crooked direction of the ‘‘ umbilical rays.” In 1856 another 
species was described by the late Professor Bailey (‘ Amer. 
Journ. of Science and Arts,’ vol. xxii, p. 1), and named 
by him Brooket in compliment to Lieutenant Brooke of the 
U.S. Navy, who obtained it from a depth of 1700 fathoms 
in the Sea of Kamtschatka. Professor Bailey derives his 
specific character from the ‘umbilical rays,” placing no 
dependence whatever on number, either in the present genus or 
in the “allied forms of Asterolampra, Heliopelta, Actinoptychus, 
Actinocyclus, &e.” 
If this view regarding number be correct, and it is now 
being very generally adopted, the Antarctic species of 
Asteromphalus will probably have to be reduced ; but in the 
absence of authentic specimens it is impossible to speak with 
any certainty. In his ‘ Mikrogeologie,’ tab. xxxv (xxi), 
Ehrenberg has figured four out of his seven species, and it 
is remarkable that in every instance the ‘“ umbilical rays” 
are represented as all given off from the very extremity of 
the ‘‘ base of the median line,” or, if De Brébisson’s view be 
received, who does not recognise ‘umbilical rays,” the 
enlarged bases of the rays meet at one point, however 
numerous they may be, and are not planted along the sides 
as well as on the apex of the base of the median ray, as is the 
case in every species more recently discovered. It would be 
interesting to know whether, in all the Antarctic species, the 
apex of the base of the median ray is precisely centrical as 
represented in the ‘ Mikrogeologie ;? and whether the “ um- 
bilical rays’ invariably arise from that point. 
Early in 1857 De Brébisson proposed his new genus 
Spatangidum (‘ Bulletin de la Société Linnéene de Norman- 
die,’ vol. 11), for the reception of certain forms nearly allied 
to Asteromphalus, which he had observed in Peruvian guano. 
His character is in the following terms: 
“ SpatancipuM, Bréb.—Lorica simplex, bivalvis, suborbi- 
cularis, valvula una convexiore. Discus cellulosus vel gran- 
ulosus, uterque stelle excentrice radiantis notatus, radiis 
(ambulacris) levibus.” 
His great mark of distinction is the excentrical position of 
the rays; but I fear that this character cannot in every case 
be relied on. I have seen various discs where it was scarcely 
