328 
THE GEOLOGIST. 
The question presented to me by Mr. Flower's fossil was, whether to 
consider it part of the envelope of a new kind of Holothuria or whether 
it might be no more than a fragment of the oral disk of some great 
unknown Echinus. Portions of the imbricating scaly armour of a Psolus 
had been met with when examining the fossils of the boulder clay collected 
by Mr. J. Eichmond, of Eothsay ; but in Psolus, while the greater part of 
the body is clothed with fish-like scales, the ambulacra arc only developed 
on one side, forming a creeping disk, the scales of which are small and not 
imbricated. On the other hand, the peristome of the largest known Echi- 
nite from the Chalk is less than an inch in diameter ; and the largest 
recent sea-urchin in the museum has an oral disk not more than 2 inches 
wide, whereas the fossil is a segment of a disk which must have been 
at least 4 inches across. This objection, on the score of size, was how- 
ever less felt, because the Cyphosomas and Diademas of the Chalk have 
larger oral and apical orifices than any other urchins, and the character 
of their apical disk was unknown, being only preserved in a few minute 
specimens of C. difficile, from Chute Farm. Moreover, there were indi- 
cations in the Upper Chalk of a great Diadema, of which nothing more 
had been obtained than scattered plates and fragments of spines. This 
species is referred to in Decade Y. of the Geological Survey (Article Dia- 
dema, Section C, spines tubular, annulated). Mr. Wetherell obtained a 
mass of chalk containing above one hundred fragments of spines, which 
are hollow, striated and annulated, as in the recent D. calamaria. From 
the plates mingled with the spines we ascertained that the ambulacral pores 
presented the usual characters, being arranged in single file, and a little 
crov^ ded near the peristome ; but many of the plates presented only their 
smooth inner surfaces. A smaller mass of chalk, in Mr. "Wiltshire's 
cabinet, contains similar plates and spines, mingled with a few true scales 
and minute truncated spines like those of Echinothuria. The Diadema 
spines were erroneously referred by Prof. E. Forbes to the genus Micraster 
(decade iii. pi. 10, fig. 15 ; bad, for they are not spiral). They are also 
figured by Dixon, in his ' Geology of Sussex,' and described by Forbes as 
" spines of a Cidaris." Diademas possessing spines of this character are 
known to occur in the Upper Cretaceous strata of France ; and Dr. Wright 
has lately obtained a small specimen from the chloritic marls of Dorset- 
shire. In these the apical disk is quite small. 
A more serious difficulty, in comparing Mr. Flower's fossil with the oral 
disk of any Echinite, was presented by the arrangement of the plates ; in 
the recent Echinidae (like the Cidaris represented by Fig. E) they are all 
directed towards the dental orifice, but here the alternate series take 
opposite " dips," the ambulacral plates overlapping one way and the others 
in a contrary direction. 
Last year, while I was still hesitating about the publication of Mr. 
Flower's fossil, a second specimen was obtained from Charlton, in Kent, 
