26 
Greville^ on New Diatoms. 
Mr. Moss, of Lancaster,, who lias aided him in his investiga- 
tions, and to whom I, also, am indebted for interesting con- 
tributions to my cabinet. 
AULACODISCUS. 
Aulacodiscus gig as, n. sp., Grev. — Very large; disc with 
numerous (10) rays, and large, round, equal cellules, some- 
what distant in the centre; umbilicus a .defined, circular, 
blank space, minutely granulate; ray-furrows gradually 
widening, until they enclose the almost marginal processes; 
margin punctate. (Fig. 23.) 
Rah. Barbadoes deposit, Cambridge estate; C. Johnson, 
Esq. 
Among the finest and rarest of Mr. Johnson's many dis- 
coveries. It is at the same time an unquestionably distinct 
species. The spaces unoccupied by the large rounded cel- 
lules show a very evident granulated structure, extending 
even to the margin, which, contrary to what is usually ob- 
served in this genus, is not striated. The umbilicus is large 
and well-defined by a circle of cellules, a pair of which mark 
the commencement of each of the ten rays. The latter are 
not conspicuous as they leave the umbilicus, but soon begin 
to widen, and then become very characteristic, passing closely 
round the processes, which lie within the loop like a stone in 
the bottom of a sling. The cellules are rather sparse for 
some distance from the centre, become gradually more nu- 
merous, and for more than half the radius closely fill up the 
compartments. Diameter •0090"". 
Hemiaulus. 
Dr. P. A. C. Heiberg, in his beautiful ^ Conspectus criti- 
cus Diatomacearum Danicarum,' has proposed to make 
Hemiaulufi the type of a new family ; but the diatoms I am 
about to describe — not the least curious of the treasures of 
the Barbadoes deposit — in the present and following Series 
will probably tend to modify his views. The following 
synopsis of his characters will, I trust, accurately convey his 
ideas; and in translating it from the Danish I have to 
acknowledge the kind assistance of my friend Mr. James B. 
Davies, Assistant-Curator of the Museum of Natural History 
in the University of Edinburgh. It is right to state, how- 
ever, that for the sake of perspicuity, I have occasionally de- 
viated from the literal words of the text. 
